398 MAMMALIA [CETA.CEA. groping for them with its long snout in the muddy water at the bottom of the rivers. It attains the length of 8 feet. Inia. Teeth variable, from 26 to 33 on each side of each jaw ; those at the posterior part with a distinct tubercle at the inner side of the base of the crown. Vertebra : C 7, D 13, L 3, CIS; total 41. Transverse processes of lumbar vertebra very broad. Sternum short and broad, and consisting of a single segment only. Dorsal fin a mere ridge. The long cylindrical rostrum externally furnished with scattered, stout, and crisp hairs. One species only is known, /. geo/rensis, about 8 feet in length, inhabiting the upper Amazon and its tributary streams. Pontoporia. Teeth 50 to 60 on each side of each jaw, with a cingulum at the base of the crown. Jaws very long and slender. Vertebrae: C 7, D 10, L 5, C 19 ; total 41. Transverse processesof the lumbar vertebrae extremely broad. Sternum elongated, composed of two segments, with four sternal ribs attached. Dorsal fin rather small, triangular, pointed. External respiratory aperture transverse, FIG. 48. Pontoporia olainrillii. From Burmeister. crescentic. This genus connects the last two forms with the true Delpfiinidas. The only species, P. blainvillii, is one of the smallest of the whole order, not exceeding 5 feet in length. It has only been met with at the mouth of the Rio de la Plata, near Buenos Ayres, and there is at present no evidence that it ascends into the fresh waters of the river. Family DELPHINID.E. Teeth usually numerous in both jaws. Fterygoid bones short, thin, each involuted to form with a process of the palate bone the outer wall of the post-palatine air-sinus. Symphysis of mandible short, or moderate, never exceeding one-third of the length of the ramus. Lacrymal bone not distinct from the jugal. Transverse processes of the dorsal vertebrae gradually transferred from the arches to the bodies of the vertebrae without any sudden break, and becoming posteriorly continuous serially with the transverse pro cesses of the lumbar vertebrae. Anterior ribs attached to the transverse process by the tubercle, and to the body of the vertebra by the head ; the latter attachment lost in the posterior ribs. Sternal ribs firmly ossified. External respiratory aperture trans verse, crescentic, with the horns of the crescent pointing forwards. A very large group, closely united in essential characters but presenting great modifications in details. The different types are mostly so connected by intermediate or osculant forms that there are great difficulties in grouping them into natural subfamilies, Even the formation of well-defined genera is by no means satis factory in all cases. Monodon. Besides some irregular rudimentary teeth, the entire dentition is reduced to a single pair of teeth which lie horizontally in the maxilla, and which in the female remain permanently concealed within the alveolus, so that this sex is practically toothless, while in the male (see fig. 49) the right tooth usually remains similarly concealed and abortive and the left is immensely developed, attaining a length equal to more than half that of the entire animal, projecting horizontally from the head in the form of a cylindrical, or slightly tapering, pointed tusk, without enamel, and with the surface marked by spiral grooves and ridges, running.in a sinistral direction. (When, as occasionally happens, both tusks are developed, the spiral grooves have the same direction in each.) Vertebrae: C 7, D 11, L 6, C 26 ; total 50. Cervical region comparatively long, and all the vertebra distinct, or with irregular unions towards the middle of the series, the atlas and axis being usually free. Manus small, short, and broad ; second and third digits nearly equal, fourth slightly shorter. No dorsal fin. One species, M. monoccros, the Narwhal or Sea-unicorn, so called on account of the remarkable single, horn-like tusk of the male, which often grows to a length of 7 or 8 feet. It inhabits the Arctic Ocean, where it is tolerably abundant and gregarious, feeding on various species of cephalopods, small fish, and crustaceans. It is rarely seen south of 65 N. lat. Delphinapterus. This genus is closely allied to the last in external form, as well as anatomical structure, differing only in the very different character of the dentition. Teeth from to , occupying the anterior three-fourths of the rostrum and correspond ing portion of the mandible, rather small, conical, and pointed when unworn, but usually become obliquely truncated, separated by intervals considerably wider than the diameter of the tooth, and implanted obliquely, the crowns inclining forwards especially in the upper jaw. Skull rather narrow and elongated, depressed. Premaxillae convex in front of the nares. Rostrum about equal in length to the cranial portion of the skull, triangular, broad at the base, and gradually contracting towards the apex, when it is some what curved downwards. Vertebrae: C 7, D 11, L 9, C 23 ; total 50. Cervical vertebrae free. Manus broad, short, arid rounded, all the digits being tolerably well developed, ex cept the first. Anterior part of head rounded ; no distinct snout. No dorsal fin, but a low ridge in its place. One species, D. leucas (fig. 50),the Beluga, or White Whale, so called from its pure white colour, about 12 feet long, abundant in the Arctic seas, and extending as far south on the American coast as the river St Lawrence, which it ascends for a con siderable distance. On rare occasions it has been seen on the coast of Scotland. In all the remaining genera of DeJphi- nidee the cervical region of the vertebral column is very short, and the first two, and usually more, of the vertebrae are firmly united. Phocsena. Teeth -f f to f f , small, oc cupying nearly the whole length of the rostrum, with compressed, spade-shaped crowns, separated from the root by a constricted neck. Rostrum rather shorter than the cranium proper, broad at the base and tapering towards the apex. Premaxillfe raised into tuberosities in front of the nares. The frontal bones forming a somewhat square, elevated pro tuberance in the middle line of the skull behind the nares, rising altogether above the flattened nasals. Symphysis of man dible very short. Vertebrae: C7, D 13, L 14, C 30; total 64 (subject to slight individual modifications). First to sixth cervical vertebrae, and sometimes the seventh also, coalesced. Manus of mode rate size, oval, slightly falcate ; second and third digits nearly equal in length ; fourth and fifth well developed, but shorter. Head short, moderately rounded in front of the blowhole. Dorsal fin (in the typical species) near the middle of the back, triangular ; its height considerably less than the length of the base ; its anterior edge frequently furnished with one or more rows of conical horny tubercles. The common Porpoise or Porpus, P. communis, is the best known of British Cetaceans (see PORPOISE). A species from Japan, P. melas, closely allied in osteological and dental char acters, but which wants the dorsal fin, constitutes the genus Neomeris of Gray. It is entirely black in colour, and has but r | teeth, rather larger proportionally than those of P. communis, but of similar form. Orcella. Teeth ^-f to , small, conical, pointed, rather closely set, and occupying nearly the whole length of the rostrum. Skull sub-globular, high. Rostrum nearly equal in length to the cranial portion of the skull, tapering. Mantis of moderate size, not elongated, but somewhat pointed. All the bones of the digits broader than FIG. 49. Upper Surface of the long, except the proximal phalanges of Skull of Male Narwhal the index and third fingers. Head globular in front. Dorsal fin rather small, placed behind the middle of the body. Two species, both of small size 0. brcvirostris, from the Bay of Bengal, and 0. fluminalis, from the Irraw r addy river, from 300 to 900 miles from the sea. Our present knowledge of the anatomy, geographical distribution, and habits of these interesting Cetaceans is almost entirely due to the researches of Dr J. Anderson. 1 Orca. Teeth about ff, occupying nearly the whole length of the rostrum, very large and stout, with conical recurved crowns, and large roots, expanded laterally and flattened, or rather hollowed, on the anterior and posterior surfaces. Rostrum about equal in length to the cranial part of the skull, broad and flattened above, rounded in fro it ; premaxillfe broad and rather concave in front of the 1 Anatomical and Zoological Researches, comprising an Account of the Zoological Results of the two Expeditions to Western Yunnan, in 1SG8 and 1875 (1878). (Monodon monocerof), with the whole of both teeth ex posed by removal of the upper wall of their alveolar
cavities.