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the common fences : they are compofed of flender filaments, not exceeding the twentieth part of an inch in length ; and thefe were the more eafy to be overlooked, as they are of the fame colour with the leaves of the plant. Thofe plants on which Mr. Reaumur found thefe flowers, were not, however, entirely covered with them, but they flood at about a twelfth of an inch diftance from one another : it was in the month of July that Mr. Reaumur examined this plant ; and at that time there were no feeds difcoverable upon it ; but doubt- lefs at a proper feafon, and with proper care in the obferver, they will hereafter be difcovered as plainly in this as in many other of the fea-plants, ufually fuppofed by authors to have wanted them. Mem. Acad. Par. 1 7 1 2.
POLYTHALAMIUM, in natural hiftory, a name by which fome authors have called the tubuli mar'tni concamerati, a fort of fea-fhell found frequently foflile, with other fhells, in Swe- den, and brought over to us in the ftones ufed for pavements, but not known in its recent ftate.
It is of the fame general ftructure with the cornu ammonis, and thick nautilus, being compofed of feveral cells or ca- vities, communicating with one another, by means of a fiphunculus or pipe ; but it is ufually ftrait, tho' fometimes its end is twifted exactly in the manner of the cornu ammonis.
POLYTHALAMIUS, in natural hiftory, a term invented by Breynius, to exprefs a clafs of fhells, the character of which is, that they are hollow, fhelly bodies, either ftrait or regu- larly twifted into a fpiral form, always wide at the mouth, and growing narrower to the other extremity ; they are di- vided within into feveral cells or chambers, which are called thalamic each feparated from the other by a diaphragm, or partition of fhelly matter. The upper or largefr. chamber, contains the body of the animal, but all the others are per- forated by a fiphunculus, which gives them communication one with another, and which runs from the mouth to the very apex, growing flenderer all the way.
Of this clafs he diftinguifhes four genera, the orthoceros, li- tuus, ammonites, and nautilus, each of which fee under their feveral heads. The two laft of thefe, he obferves, have been long well known to the world j the two firft he gives as new, and of his own difcovery.
Of the orthoceratitx, which are ftones caft in the fhells of the orthoceros, there are at prefentdiftinguifhed nine kinds : thefe differ from one another principally in the pofition of the fiphun- culus, or in their external form ; the cone which they defcribe, running quicker or more gradually to a point; or, finally, by the thalami or chambers being wider or narrower. Breynius de Polythalam.
POLYTHRIX, the hair-Jlone, a name given by fome writers to thofe German agates which have received into their mafs, while yet foft, either the branches of the confervse, or other capillary water-plants, or elfe have thin fireaks of a coarfer matter frequent in them, and running in the form of hairs.
POLYTRICHUM, in botany, the name of a genus of mofles, the characters of which are thefe : The ftalks are not much branched, and the capfules have calyptrs to cover them till mature. This calyptra is always hairy, and in many of the fpecies is compofed merely of long hairs, ranged lengthwife together, without any membrane; the others have membra- naceous calyptrae, but covered with erect hairs. The leaves of thofe kinds are rigid, and have a membranaceous appen- dage, by which they touch the ftalks and furround them ; thofe of the others are fofter, and have not this, appendage. The bafis of the pedicle which fupports the head, is alfo in thefe furrounded by a fort of tube, round which there ftand a number of membranaceous leaves ; this is more vifible in the larger than in the fmaller fpecies. The capfules in fome of thefe mofies are fquare, in others they are roundifh ; and the fquare ones have ufually an apophyfis, by which they are joined to the pedicle, which the round ones never have, or atleaft very feldom. See Tab. of MofTes, N° 12, The polytricbum of the firft order, are thofe which have fquare capfules, which are joined to the pedicles by a round apophy- fis ; of tl-.efe the following are all the known fpecies : 1 . The common large, fquare -headed, great-leaved polytricbum, or great goldilocks. This is very common in woods, in boggy places, and on heaths. 2. The IefTerand ftiffer fquare-headed juniper-leaved heath-polytricbum. The leaves of this are fhort, and the whole plant much fmaller than the former. It is very common on heaths. 3. 1 he fmall hairy fquare-headed poly- Jrkhum\ the leaves of this are of a bluifh green in winter, and of a reddifh brown in fummcr. It grows in dry barren places.
The fecond order of the polytricbum comprehends thofe which have round capfules, and ufually want the apophyfis, by which the others are joined to the ftalk. The following are all the known fpecies of this kind : 1. The branched alpine polytri- cbum, bearing heads from its tops. The leaves of this are narrow and fomcwhat hollowed, and when viewed by a mi- crofcope, appear ferrated at the edges. It grows on the Welch mountains. 2. The branched polytricbum, bearing heads from its fides. This grows to two inches high, and is found in the fame places with the former. 3. The dwarf round-head- ed aloe-leaved polytricbum. This has fcarce any ftalk, but lies upon the ground ; it is frequent by way fides, and ripens its Suppl. Vol. II,
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capfules early in the fpring. 4. The dwarf long-headed aloe- leaved polytricbum. The ftalks of this are a little longer than thofe of the former. It grows in wet places on heaths c The larger fhort-fhanked fslytricbum. This is branched ' tho' very low, and refembles at firft fight one of the bryums, mow- ing in round tufts. The leaves are of a deep green, and very pellucid ; they have a middle rib vifible in them, and are dif- poled in fomewhat of a ftellate manner at the ends of the branches. It grows in woods on the barks of old trees, and fometimes on walls. 6. The leffer fhort-fhanked polytricbum. This is much fmaller than the former, and produces its cap- fules a little later, it grows on trees and walls, and fome- times on the earth. 7. The fhort-fhanked polytricbum, with ftrait and plaited leaves. This grows in tufts. Its ftalks are an inch high, and its leaves of a dark green. It grows on rocks and old walls. 8. The fine-leaved curled polytricbum, with fliarp hairy calyptrse. This is found in tufts compofed of thick branches, furrounded with very numerous leaves, each ending in a hair at the end. It grows in woods, and ripens its capfules in March and April. 9. The dwarf fine- leaved polytricbum, with cylindric heads. This grows on fandy places in north America. Bitten. Hift. Mufc. p. 430. Old writers are very full of the praifes of this plant for its virtues in making the hair grow thick ; but this feems to have been an opinion taken upon no better reafon than the like- nefs of the ftalks of the plant to hairs. They make a de- coction of it in fome parts of Germany, and wafh the head carefully with it in the increafe of the moon; but there is no great reafon for fuppofing this liquor does any thing more than warm water would do.
POLYTROPHEROS, a name given by the Greeks to coarfe bread, from its conveying much more nourifhment to the body than the finer kind ; which they called, by way of dis- tinction, oligotrophies.
POLYZONOS, iri natural hiftory, a name given by the an- tients to a fpecies of onyx, which had a dark or blackifh ground, with a great number of white zones.
pOMA, in the writings of the old Greek phyficians, a word which has given the commentators a great deal of trouble rightly to explain. Diofcorides, in defcribing the onyx indi- cus, which we improperly call blatta byzantia, tells us that it was like the part of that fhell ufually called by the name of byzantine blatta, but that it was in reality the poma of another fhell.
Some have underftood Diofcorides, as if he meant that the whole fhell of the fifh, which he defcribes as being like the purpura, was what he called onyx indicus ; but it is not eafy to fay why fo correct an author fhould call a whole fhell poma conchyli, nor what refemblance there could be between a fhell like the purpura and a human nail, onyx ? Actuarius, and many others, fenfible that a part of a fhell only was meant by this name, have translated it the os naji ; but the bone of the nofe of a fhell-fifh is fo odd an expreffion, that one fcarce knows what to underftand by it.
The purpura of the antients has a long tongue, with which it pierces the fhells of the chamse, and other fifh that It feeds on ; but whether this or the operculum, which all fhells of this kind have, is meant by the word, is not to be afecrtained by this phrafe os nafi.
The true meaning of the word is to be made out before any thing can be judged of what is meant to be exprefi'ed by it here, where it is evidently ufed in a metaphorical fenfe. Braflavolus fays, that it means all forts of fhelly or cruftace- ous coverings of fifties ; but this is an abfolute error; for even the medical authors do not extend its fenfe fo far as that. The Greeks called all fhells by the name ojlraca, and this author attributing a different virtue to the whole fhell of the purpura, fufficiently fpeaks this poma to have been but a part of it. The Arabian writers were fenfible of this, and rendered it by the words adfor althaib, which fignify fragments like nails. Serapio calls it mukatha, which fignifies a morfel or piece of any thing cut from a folid body. The original fenfe in which the Greeks ufed the word poma, was to exprefs the thing that clofed the mouth of any vefi'd with a long and narrow neck. The Latins expreffed this by the word sperculwn, and fome- times, as in Sulpicius Severus, by the word umbo. The antients alfo ufed the word poma to exprefs the lid or covering of a well, or of any hole. Now the purpura buc- cina, and all thofe other fhells which the Greeks have called firom'oide, may in fome fort be compared to narrow arid deep vefiels, and they have all of them a iort of poma; or, as the Latins have exprefied it, an operculum, to ftop the mouth of the fhell, and prevent the ingrefs or egrefs of 'any thing at the creature's pleafure. This operculum being what Ariftotle has called epicalymma, is certainly aifo what Diofcorides means by the word poma ; Ariftotle has been very exprefs in his defcrip- tion of it, and fays that the tongue lies under it, and that all the Jlromboide fhells gave it. The cnyx indicus, therefore, Was only the pmia, or operculum of an Indian jhow bride fhell, of the bucctnum or purpura kind, and was of a fweet fmell, and thence had the term aromaticus, or odoratus. 'The name onyx is eafily accounted for, as the poma of every fhell of this kind is flat and thin, ajid does not unaptly refemble the human nail
a U u. PQMATIA,
