II. Pinnipedia
The second suborder is formed by the seals, walruses and eared seals, which differ from the rest of the Carnivora mainly in the limbs being modified for aquatic progression; the two upper segments being very short and partially enveloped in the general integument of the body, while the third, especially in the hind extremities, is elongated, expanded and webbed. There are always five well-developed digits on each limb. In the hind-limb the two marginal digits (first and fifth) are stouter and generally larger than the others. The teeth also differ from those of the more typical Carnivora. The incisors are always fewer than 3⁄3. The chsek series consists generally of four premolars and one molar of uniform characters, with never more than two roots, and with conical, more or less compressed, pointed crowns, which may have accessory cusps, placed before or behind the principal one, but are never broad and tuberculated. The milk-teeth are small, simple and shed or absorbed at an early age, usually either before or within a few days after birth. The brain is relatively large, the cerebral hemispheres broad in proportion to their length, and with numerous and complex convolutions. There is a very short caecum; the kidneys are divided into numerous distinct lobules. There are no Cowper’s glands. Teats two or four, abdominal. No clavicles. Tail always short. Eyes large and exposed, with flat cornea. The nostrils close by the elasticity of their walls, and are opened at will by muscular action.
The members of this group are aquatic, spending the greater part of their time in the water, swimming and diving with great facility, feeding mainly on fish, crustaceans and other marine animals, and progressing on land with difficulty, but always coming on shore for the purpose of bringing forth their young. They are generally marine, but occasionally ascend large rivers, and some inhabit inland seas and lakes, as the Caspian and Baikal. Though not numerous in species, they are widely distributed over the world, but occur most abundantly on the coasts of lands situated in cold and temperate zones.
As mentioned in the article [Creodonta], the true seals (Phocidae), together with the walruses, may be directly descended from the primitive Creodont Carnivora. The eared seals, on the other hand, show signs of affinity with the bears; but as they are of earlier geological age than the latter, they cannot be derived from that group.
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 5⁄5 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. 4⁄4, m. 1⁄1, 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 3⁄2. 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. 3⁄2, c. 1⁄1, p. 4⁄4, m. 1⁄1; 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 2⁄2; 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. 2⁄2, c. 1⁄1, p. 4⁄4, m. 1⁄1; 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.