Popular Science Monthly/Volume 9/May 1876/The Mollusks of the Rocky Mountains
|THE MOLLUSKS OF THE ROCKY MOUNTAINS.|
IN the summer of 1874 it was my privilege to accompany one of the parties of the United States Geological Survey of the Territories, of which Dr. F. V. Hayden is chief. The field of operations was the mountainous region of Southern Colorado, and it afforded a good opportunity to examine the natural history of the region traversed.
The mammals of the Rocky Mountains have long been well known, particularly the large game, which, except in the distant portions of the Territory of Colorado, has been greatly depleted by the constant pursuit of hunters and trappers. The case is somewhat the same with the game-birds; while the enthusiastic labors of Henshaw, Aiken, Allen, Coues, and other ornithologists, have given us a very complete knowledge of all the birds and their habitats. The fishes and reptiles have received some attention too; and, in the lower, invertebrate forms of life, the investigations of Thomas upon the grasshoppers, Carpenter on the butterflies and moths, and Edwards, Packard, and Hagen on other insects, and the reports upon Crustacea and worms by Verrill, Smith, Leidy and others, have given us a tolerable knowledge of the extent to which those forms are to be found in that region. But the mollusks of the mountains—land-snails, pond-snails, river-snails, and fresh-water mussels—have been almost entirely neglected, except by Dr. Cooper, in the north. From Colorado only seven had been reported, which were collected by Lieutenant Carpenter. This, then, seemed to be the field most needing cultivation, and my attention was chiefly turned to it during three months of wandering over the mountain-ranges, parks, and sterile plains, that diversify the country between Middle Park and the corner of Arizona. Something was found at nearly every camp, and, when the collection was at home and counted, it was somewhat surprising to find over fifty species, only nine of which had been hitherto known to exist in the Central Province, where an extreme "paucity of species, . . . . owing to the nature of its climate and soil," had been alleged. Five of these species were new to science, and have since been described in the "Bulletin of the United States Geological Survey," second series, No. 2, which has since been reprinted in an extended and revised form, in the Annual Report of the Survey for 1874.
The Central Province alluded to above is the name given by Mr. W. G. Binney to that portion of the United States embraced between the crests of the Sierra Nevada and Cascade Mountains on the west and the edge of the great plains on the east. It was considered to be unfavorable to the development of pulmonates and deficient in the number of species to be found, and that its fauna was closely allied to that of the Eastern States, whence it had been largely derived by way of the north, where the plains are succeeded by forests and the Rocky Mountains dwindle into hills.
With respect to this distribution of mollusks in Colorado, none were found on the eastern slope of the range, although there is no conclusive evidence that they do not exist there; altitude seemed to have very little influence upon their dispersion, as long as other favorable conditions were present, and some species had a very local distribution.
The eastern slope of the Snowy Range is abrupt, and receives comparatively little rain. Westward of the summit, however, certain genera—as Zonites, Vitrina, Vallonia, Patula, Pupa, Succinea, and Pisidium—were everywhere represented. Vitrinas and pupas were, perhaps, the most common forms, the latter being particularly numerous on the Sierras in the southeastern corner of the Territory, where Pupilla alticola were traced up to the very limit of timber-growth, and upon the face of precipitous cliffs of volcanic rock, in whose clefts only tufts of grass could gain a foothold. With the latter shell also occurred some small succineas, and a mollusk with a delicate, box-shaped shell, only one-tenth of an inch in diameter. Plenty of these little fellows, as lively as could be, were to be found at the astonishing height of 11,500 feet. They proved to be undescribed, and to belong to the sub-genus Microphysa, the two American species of which, heretofore known, are natives of the Gulf coast and the West Indies. Why this species should depart so far from the habits of its congeners as to thrive best in the arctic climate of these mountaintops, is strange. This Microphysa was afterward met with in the valleys south of these Sierras, and in the mountains west of North Park. In this same southern group of mountains many other shells were found—at a lesser altitude, but where water froze every night in August—of the same species as existed in other parts of the Territory, and, indeed, all over the Central Province. The finding of Papilla Blandi, heretofore known only as a fossil in Missouri River drift, living and abundant, is an instance worthy of special mention.
It would seem, then, that a range of high mountains, or any number of ranges would not offer a serious obstacle to the migration of land mollusks, or an insurmountable one to fresh-water forms. The widespread dissemination of such slow-moving creatures is a curious argument for the length of time that the country must have remained in substantially its present condition.
The Sierras of which I have spoken are those which encircle Baker's Park and the San Juan mining region, and extend westward to the base of the great Uncompahgre Mountains, which trend northward not far from the Utah line. This group of volcanic and quartzite peaks constitutes the highest land anywhere in that region, and gives source both to the Rio Grande del Norte and to the head-waters of the Great Colorado River. Its steep southern sides are gashed with tremendous gulches through which the Rio las Animas, the Rio La Plata, the Rio los Mancos, and other streams, which go to make up the Rio San Juan, flow out into the terrible cañon-cut deserts that stretch away across Arizona to the Gila River. For a few miles after emerging from their rocky gates, these rivers water beautiful and fertile valleys, which are cut through the sandstones upturned against the intruded peaks, and which abound in springs. In these valleys are plenty of timber and undergrowth, the climate is rarely cold enough for snow even in winter, and there I expected to gather a rich conchological harvest. In this I was not disappointed, only regretting that I could not make a more thorough examination than was permitted by the rapidity of our travel. Between the Animas and La Plata the trail passes through a valley between the lowest of the foot-hills, where there is a pond of several acres extent, resorted to by all sorts of wildfowl, inhabited by many forms of amphibious life, and choked with an exuberant aquatic vegetation. Here were found thousands of limneas of several species, and quantities of the common Planorbis trivolvis showing a large range of variation among themselves. Like the limneas, the planorbs were extremely fragile in texture, which may be owing partly to the soft bottom, and partly to the scarcity of lime in the water; and they were distinguished by a short vertical diameter, which peculiarity, also, may have been acquired by them from the necessities of their habitat, since snails having shells with small breadth of beam could most advantageously pass between the stalks of standing water-plants that everywhere crowded the pond. But the astonishing fact about this pond was, that on the shore were found perfect specimens—although dead—of the marine genus Truncatella, a broken specimen of an Arca, and living crabs pronounced by Prof. Sidney I. Smith, of New Haven, to be true salt-water forms belonging to the family Astacidæ, That these are survivors of the period, probably comparatively recent, when here was a soft-water marsh that remained caught in this basin among the hills after the country, for a long distance south of it, had become dry land, seems very evident. It is difficult otherwise to account for their presence.
Farther on, in the valley of the Rio La Plata, where it emerges from its magnificent quartz cañon, and where the gold placer-mines and prospective city of La Plata are situated, a fine collecting-ground was found. This was so far south that many deciduous trees grew in the river-bottoms, and nearly every terrestrial species hitherto met with was there to be had in plenty. For the next ten days we were entirely in the lava-blasted, treeless and waterless deserts on the northern margin of the Rio San Juan, engaged in exploring the vestiges of that ancient semi-civilized race of village Indians, the remnants of which still exist in the small tribe of Moquis on the Little Colorado. During this time no mollusks were found except, where there was a little moisture, a few pupas, which seem able to live anywhere, and many bleached shells of various species that had been drifted down from the mountains at times of high water.
Our return-journey from the San Juan country was made from its very sources along the course of the Rio Grande. It led us through Antelope Park, on the eastern side of which lies St. Mary's Lake, a beautiful little sheet of crystal water studded with islands, and held among precipitous cliffs that afford it no visible outlet. It seems to be merely a great rocky basin, holding the melted snows of the surrounding heights. Its surface is over 9,000 feet above the sea. There existed in countless numbers in this lake a large species of coil-shell which was a nondescript, and which I have since named Helisoma plexata. Each of the hundreds of individuals seen possessed in a more or less marked degree a twisted appearance, resulting from a change in the plane of revolution in old age, which is the most striking specific character. This sudden change in the directness of the growth causes the carina of the third whorl to rise into a sharp shoulder on the right side, while on the opposite side the third whorl sinks underneath the overflowing outer whorl. A similar change often occurs in the fourth whorl, giving a braided look to the shell. How this species came almost alone to inhabit this secluded lake is a problem, complicated by the fact that probably there is not another large Planorbis within fifty miles. That the wild-fowls, abundant on the lake, brought the eggs clinging to their feet, may be a plausible explanation; but where did they bring them from, and when? The bottom of the lake is, for the most part, rough conglomerate rock, and is in many places filled with tangled water-plants, which may partially account for the peculiarities of the species. The shells of this genus appear to be especially subject to distortion under abnormal conditions.
Continuing our course down the valley of the Rio Grande to the town of Del Norte, we there left the river and struck across the San Luis Valley to Mosca Pass through the Sangre de Cristo Range. This alkali and sage-brush plain, fifty miles wide, is very far from being "the garden of the world," as it has been styled. Near the eastern side is a group of lakes, the water of which is highly alkaline. These lakes are the abode and breeding-place of wild geese and ducks in the greatest number, which are tormented without end by the gulls that also make the lakes their home. On the gravelly beaches I picked up many shells, and doubtless in the deep water many more species might have been dredged, had there been time. But nowhere were there any bivalves, except the little cyclades. The fact that there was no lack of molluscan life in these intensely bitter waters was not surprising, since mollusks seem to flourish in mineral springs of both hot and cold water everywhere. We had seen before a fine illustration of this adaptation to peculiar conditions. The Grand River, which flows through Middle Park, contains no mollusks at all that I could discover; but at Hot Springs, in a little lagoon filled at high water, large, clear, ampullacea-like forms of the familiar Physa heterostropha were common. Close by, in the few yards of exposed outlet of the springs of hot sulphur-water from which the locality derives its name, there occurred in the greatest profusion a blackish, globose variety of the same species only one-fifth of an inch long. The temperature of this water was at some points as high as 100° Fahr. In the basin of a still hotter spring not ten feet away, whose waters were saturated with chlorides of sodium and magnesium, hundreds of still smaller Physæ were-floating about in mats glued together by a tangle of confervoid vegetation and the depositions of the water. All these seemed to have lost their apices by erosion, "which is extremely liable to happen to shells living in water charged with alkaline salts other than lime." On the other hand, quite as small and black were the examples from the pure cold springs near Saguache, where there was seemingly nothing whatever to stunt their growth.
I was stimulated, by the results of my study of my own collection from Colorado, to gather all possible information about the mollusks of the Central Province generally, as it has been limited above. The bibliography was quite large, but the notes of locality and station very meagre. Tabulating the sum of the information open to me, and including my own summer's work, I found that 138 nominal species had been recorded as occurring in this inter-montanic region. Of these, 49 were also Californian species; 15 occurred also in the Eastern United States; 8 hailed from the Colorado Desert; 7 were found all over the continent, and 8 all over the world; and 3 belonged in the Eastern Province, west of the Alleghanies only. This left 47 nominal species, whose range, so far as yet known, is confined to the Central Province. Many of the specific names in this list, however, rest upon very insecure foundations, and will, no doubt, soon be reduced to synonyms. With respect to their vertical distribution, observations in the Rocky Mountains do not tally well with D'Orbigny's notes from the Andes, since out of 156 species discovered in South America, he found only 13 between the thirty-fourth and forty-fifth parallels of latitude—which corresponds to the district of north latitude considered here—and only 10 species were found above 5,000 feet. My list of the Rocky Mountain mollusca, on the contrary, shows that 55 species out of the 138 inhabit heights exceeding 5,000 feet, and 10 species have been found above 10,000 feet. These latter, however, are all recorded from mountains south of the thirty-ninth parallel; but it is safe to say that, where there is moisture, a goodly collection of mollusks can be made in the mountains of the Territories all the way up to the timber-line. This is probably true of all parts of the world.
In a recent paper on the hypsometric distribution of mollusca in Europe, communicated to the French Academy of Sciences at Paris, at their meeting on October 11, 1875, M. P. Fischer alludes to the great regularity with which plants thrive on mountains, each at a certain height. The terrestrial mollusca, being unprovided with means of locomotion enjoyed by birds and insects, and being, moreover, dependent upon vegetable life for food, could not fail to be discovered in the same way as plants, and this supposition he confirmed by observation. Each species extends to an altitude the limits of which it does not overstep. M. Fischer has verified this in the central Pyrenees as well as in the Alps, and divided the altitudes into five zones, comprised between 1,500 feet and 7,500 feet. Each zone is distinguished by the name of a species of Helix. Thus, in the Pyrenees, the first zone, ending at a height of 3,000 feet, is called that of Helix carthusiana; the second, ending at 3,600 feet, Helix aspersa; the third, terminating at 4,500 feet, Helix limbata; the fourth, limited at 6,000 feet, Helix nemoralis; and the fifth, ending at 7,500 feet, Helix carascalensis. In the Alps, at the same altitudes, the names of the zones are respectively Helix carthusiana, obvoluta, Fontenelli, sylvatica, and glacialis. A few individual mollusks will, indeed, climb as high as 9,000 feet, but they all stop at the limit of perpetual snow. Various genera of fluviatile mollusks do not ascend higher than 3,000 feet, a circumstance which the author considered of some importance to geologists, since it proves that in the quaternary beds the fossiliferous strata containing those genera, such as Neritina, Paludina, etc., were deposited at small altitudes. The Lake of Goube, about three hours' walk from Cauterets, 5,364 feet above the level of the sea, is thickly peopled with trout, frogs, and mollusks.
The results of this inquiry into the geographical distribution of mollusks in the mountainous West are meagre enough, but may be of some use in future investigations. Whether this central region is a true zoölogical province considered with reference to the mollusca, and what is the origin of its fauna, are hardly to be answered yet. Enough seems to be known, however, to show that this inter-montanic region is not so deficient as has been supposed, either in the number of its species or in representatives of adjoining faunas. The impression that the Central Province is unfavorable to pulmonate growth also seems wrong, except in respect to the scarcity of lime in the soil, to which cause we may probably attribute the fact that the more minute forms are in large majority.
- In the "Bulletin of the Museum of Comparative Zoölogy" (Cambridge, Mass.), vol. iii.,No. 9, "Geographical Distribution of North American Mollusca."