The true seals (family Phocidae) are the most completely adapted for aquatic life of all the Pinnipedia. When on land the hind-limbs are extended backwards and take no part in progression, which is effected by a series of jumping movements Seals. produced by the muscles of the trunk, in some species aided by the fore-limbs. The soles of the feet are hairy. There is no pinna to the ear, and no scrotum, the testes being abdominal. The upper incisors have simple, pointed crowns, and vary in number in the different groups. All have well developed canines and 5 teeth of the cheek series. In those species of which the milk-dentition is known, there are three milk molars, which precede the second, third, and fourth permanent molars; the dentition is therefore p. 4, m. 1, the first premolar having as usual no milk predecessor. The skull has no post-orbital process and no alisphenoid canal. The fur is stiff and adpressed, without woolly under-fur.
In the typical group, or subfamily Phocinae, the incisors are 3. All the feet have five well-developed claws with the toes on the hind-feet subequal, the first and fifth not greatly exceeding the others in length, the interdigital membrane not extending beyond them. In the genus Halichoerus the dentition is i. 3, c. 1, p. 4, m. 1; total 34. Molars with large, simple, conical, recurved, slightly compressed crowns, having sharp anterior and posterior edges, but without accessory cusps, except sometimes the two hinder ones of the lower jaw. With the exception of the last one or two in the upper jaw and the last in the lower jaw, all are single-rooted. Vertebrae: C. 7, D. 15, L. 5, S. 4, Ca. 14. Includes only one species H. grypus, the grey seal of the coasts or Scandinavia and the British Isles.
In Phoca the dental formula is as in the last, but the teeth are smaller and more pointed. Molars with two roots (except the first in each jaw). Crowns with accessory cusps. Vertebrae: C. 7, D. 14-15, L. 5, S. 4, Ca. 11-14. Head round and short. Fore-feet short with five strong, subcompressed, slightly curved, subequal, rather sharp claws. On the hind-feet the claws much narrower and less curved. The species of this genus are widely distributed throughout the northern hemisphere, and include P. barbata, the bearded seal; P. groenlandica, the Greenland seal; P. vitulina, the common seal; P. hispida, the ringed seal of the north Atlantic; P. caspica, from the Caspian and Aral Seas; and P. sibirica, from Lake Baikal. (See Seal).
The members of the second subfamily, Monachinae, have incisors 2; and the molars two-rooted, except the first. On the hind-feet the first and fifth toes greatly exceeding the others in length, with nails rudimentary or absent. In the genus Monachus, the dentition is i. 2, c. 1, p. 4, m. 1; total 32. Crowns of molars strong, conical, compressed, hollowed on the inner side, with a strongly-marked lobed cingulum, especially on the inner side, and slightly developed accessory cusps before and behind. The first and last upper and the first lower molar smaller than the others. Vertebrae: C. 7, D. 15, L. 5, S. 2, Ca. 11. All the nails of both fore and hind feet very small and rudimentary. Represented by M. albiventer, the monk-seal of the Mediterranean and adjacent parts of the Atlantic, and the West Indian M. tropicalis.
The other genera of this section have the same dental formula, but are distinguished by the characters, of the cheek-teeth and the feet. They are all inhabitants of the shores of the southern hemisphere.
In Ogmorhinus all the teeth of the cheek-series have three distinct pointed cusps, deeply separated from each other, of which the middle or principal cusp is largest and slightly recurved; the other two are nearly equal in size, and have their tips directed towards the middle one. Skull much elongated. One species, O. leptonyx, the sea-leopard, widely distributed in the Antarctic and southern temperate seas. In Lobodon the molars have compressed elongated crowns, with a principal recurved cusp, rounded and somewhat bulbous at the apex, and one anterior, and one, two or three posterior distinct accessory cusps. One species, L. carcinophagus, the crab-eating seal. In the third genus, Leptonychotes, represented by L. weddelli, the molars are small, with simple, subcompressed, conical crowns, and a broad cingulum, but no distinct accessory cusps. Finally in the white seal (Ommatophoca rossi) all the teeth are very small, those of the cheek-series with pointed, recurved crowns, and small posterior and still less developed anterior accessory cusps. Orbits very large. Nails rudimentary on front and absent on hind-feet. The skull bears a considerable resemblance to that of the next subfamily.
The presence of two pairs of upper and one pair of lower incisors is characteristic of the members of the subfamily Cystophorinae, in which the teeth of the cheek-series are generally one-rooted. The nose of the males has an appendage capable of being inflated. First and fifth toes of hind-feet greatly exceeding the others in length, with prolonged cutaneous lobes, and rudimentary or no nails. In the typical genus Cystophora the dentition is i. 2, c. 1, p. 4, m. 1; total 30; the last molar having generally two distinct roots. Beneath the skin over the face of the male, and connected with the nostrils, is a sac capable of inflation, when it forms a kind of hood covering the upper part of the head. Nails present, though small on the hind-feet. Represented by C. cristata, the hooded or bladder-nosed seal of the Polar Seas. In Macrorhinus the dentition is numerically the same as in the last, but the molars are of simpler character and all one-rooted. All the teeth, except the canines, very small relatively to the size of the animal. Hind-feet without nails. Vertebrae: C. 7, D. 15, L. 5, S. 4, Ca. 11. Nose of adult male produced into a short tubular proboscis, ordinarily flaccid, but capable of dilatation and elongation under excitement. One species, M. leoninus, the elephant-seal, or “sea elephant” of the whalers, the largest of the whole family, attaining the length of nearly 20 ft. Formerly abundant in the Antarctic Seas, and also found on the coast of California.
The next family is that of the walruses, or Odobaenidae, the single generic representative of which is in some respects intermediate between the Phocidae and Otariidae, but has a completely aberrant dentition. Walruses have no external ears, as Walrus in the Phocidae; but when on land the hind-feet are turned forwards and used in progression, though less completely than in the Otariidae. The upper canines are developed into immense tusks, which descend a long distance below the lower jaw. All the other teeth, including the lower canines, are much alike, small, simple and one-rooted, the molars with flat crowns. The skull is without post-orbital process, but has an alisphenoid canal. In the young the dentition is i. 3, c. 1, p. and m. 5, but many of these teeth are, however, lost early or remain through life in a rudimentary state, concealed by the gums. The teeth which are u0; total 18. Vertebrae: C. 7, D. 14, L. 6, S. 4, Ca. 9. Head round. Eyes rather small. Muzzle short and broad, with a group of long, very stiff, bristly whiskers on each side. The remainder of the hair-covering very short and closely pressed. Tail rudimentary. Fore-feet with subequal toes, carrying five minute flattened nails. Hind-feet with subequal toes, the fifth slightly the largest, with cutaneous lobes projecting beyond the ends as in Otaria; first and fifth with minute flattened nails; second, third and fourth with large, elongated, subcompressed pointed nails. The two species are Odobaenus rosmarus, of the Atlantic, and the closely allied O. obesus, of the Pacific. (See Walrus.)
The third and last family of the Pinnipedia, and thus of existing Carnivora, is the Otariidae, which includes the eared seals, or sea-lions and sea-bears. In all these animals, when on land, the hind-feet are turned forwards under the body, and Sea-lions aid in supporting and moving the trunk as in ordinary quadrupeds. There are small external ears. Testes suspended in a distinct external scrotum. Skull with post-orbital processes and alisphenoid canal. Soles of feet naked. By many naturalists these seals are arranged in a number of generic groups, but as the differences between them are not very great, they may all be included in the typical genus Otaria. The dental formula is i. 3, c. 1, p. 4, m. 1 or 2; total 34 or 36. The first and second upper incisors are small, with the summits of their crowns divided by deep transverse grooves into an anterior and a posterior cusp of nearly equal height; the third large and canine-like. Canines large, conical, pointed, recurved. Molars and premolars usually 5, of which the second, third and fourth are preceded by milk-teeth shed a few days after birth; sometimes (as in fig. 7) a sixth upper molar (occasionally developed on one side and not the other); all with similar characters, generally single-rooted; crown moderate, compressed, pointed, with a single principal cusp, and sometimes a cingulum, and more or less developed anterior and posterior accessory cusps. Vertebrae: C. 7, D. 15, L. 5, S. 4, Ca. 9-10. Head rounded. Eyes large; ears small, narrow and pointed. Neck long. Skin of the feet extended far beyond the nails and ends of the digits, with a deeply-lobed margin. The nails small and often quite rudimentary, especially those of the first and fifth toes of both feet; the best-developed and most constant being the three middle claws of the hind-foot, which are elongated, compressed and curved.
Sea-bears and sea-lions are widely distributed, especially in the temperate regions of both hemispheres, though absent from the coasts of the North Atlantic. They spend more of their time on shore, and range inland to greater distances than the true seals, especially at the breeding-time, though they are obliged to return to the water to seek their food. They are gregarious and polygamous, and the males usually much larger than the females. Some possess, in addition to the stiff, close, hairy covering common to the group, a fine, dense, woolly under-fur. The skins of these, when