The coadaptation of the crowns of the upper and lower teeth is inure completely alternate than in any of the terrestrial Car nivora, the lower tooth always passing into the interspace anterior to its fellow in the urer jaw. In the genus Phoca proper (Ca locephalus, Coy.) typified by the common seal ( l'h. vitulina), the dental formula is, i. 33 1-1 3-3 2-2 I 1' P' = 34. The forms M. 2-2 and proportions of these teeth are shown in Pl. l32, dig. 1., of my " Odontography." The first tooth above and below presents a complete confluence of the fangs ; they are separated from the second above ; but be low they sometimes do not become free before the fourth, and sometimes the two roots are distinct in the third and second molars. In the Phoca ancllata Nills., the principal cusp of the molar teeth is com plicated with anterior and posterior smaller cusps, sometimes one in number in the upper molars ; the anterior accessory cusp is some times wanting in the first, and is rudimentary in the rest ; but usually there are two small cusps behind the principal one, and in the three or four posterior molars in the lower jaw there are sometimes two small cusps be fore and two behind the principal one.* In the Phoca caspica the upper molars have commonly one accessory cusp before and one behind the principal lobe; the lower molars have one accessory cusp before and two be hind.
In the Phoca grwnlandica the upper molars have no anterior basal cusp and only one be hind ; the lower molars have two cusps behind and one in front, except the first, which re sembles that above, and, like it, has connate fangs.
The condition of the molar teeth is nearly the same in the Phoca barbata, but the crowns are rather thicker and stronger, and the three middle ones above have two posterior basal cusps feebly indicated, the same being more strongly marked in the four last molars below.
The following genera of seals with double rooted molars (Pelagius and Stenorhynchus) have four incisors above as well as below, i. e. 2-2 . An upper view of the molar teeth in 2-2 the Hooded Seal of the Mediterranean (Pela. gins monachus) is given in my Odontography, Pl. 132. fig. 3., as when they are worn down in an old specimen ; the crowns are thick, obtuse, sub-compressed, with a well developed cingulum, a principal lobe and an anterior and posterior accessory basal lobule ; the fangs are connate in the first tooth both above and below.
The allied sub-genus (Ommatophoca)of seals of the southern hemisphere has six molar teeth on each side of the upper, and five on each side of the lower jaw, with the principal lobe of the crown more incurved. The two first molars above are closely approximated, but this may prove to be a variety.
In the Stenorhynchus the jaws are more slender and produced, and the molar teeth aro remarkable for the long and slender shape of the principal lobe, and of the accessory basal cusps. The incisors have sharp conical re
curved crowns, like the canines, and the ex ternal ones in the upper jaw are intermediate in size between the canines and the middle in cisors.
In the Stenorhynchus leptonyx each molar tooth in both jaws is trilobed, the anterior and posterior accessory curving towards the principal one, which is bent slightly back wards ; all the divisions are sharp-pointed, and the crown of each molar thus resembles the trident or fishing-spear ; the two fangs of the first molar in both jaws are connate, In Stenorhynchus serridens the three anterior mo lars on each side of both jaws are four-lobed, there being one anterior and two posterior accessory lobes ; the remaining posterior molars (true molars) are five-lobed, the prin cipal cusp having one small lobe in front, and three developed from its posterior margin ; the summits of the lobes are obtuse, and the posterior ones are recurved like the prin cipal lobe. Sometimes the third molar be low has three instead of two posterior acces sory lobes. Occasionally, also, the second, as well as the first molar above, has it fangs con nate; but the essentially duplex nature of the seemingly single fang, which is unfailingly manifested within by the double pulp-cavity, is always outwardly indicated by the median longitudinal opposite indentations of the im planted base. These slight and unessential varieties, presented by the specimens of the Saw-toothed Sterrink (Stenorhynchus serridens) brought home by the enterprising Naturalist of Sir J. Ross's Antarctic expedition, accord with the analogous varieties noticed by the best observers of the seals of our neighbour. ing seas, as, for example, Nillson.
The Grey Seal (Halichterus gryphus) of our own seas begins, by the extension of the connate condition of the two roots through a greater proportion of the molar series, to manifest a transition to the family of seals with true single-rooted molars ; the formula 3-3 of this genus is, i. c --I . , P 3-3 . 0 4 1 2-2 nz = 34. The four middle upper in cisors are close set, with pointed recurved crowns ; the lateral ones are much larger and laniariform : the canines have moderate crowns, with a sharp ridge before and behind. The crowns of the molar teeth are conical, sub compressed longitudinally and finely grooved, with an anterior and posterior edge ; those below have generally a slight notch at the fore and back part of the base. The first molars, both above and below, are the smallest, with a simple crown and a single ventricose fang ; the second and third above, and the second, third, and fourth below, have two connate roots ; the two roots are commonly distinct in the remaining posterior molars : all the roots are very thick.