PLACODUS. The cranial structure in this genus of muschelkalk reptile is closely similar to that in Simosaurus, but its proportions are different ; it is as broad as long ; the greatest breadth being behind, whence the sides converge to an obtuse muzzle ; the entire figure viewed from above being that of a right-angled triangle, with the corners rounded off. The temporal fosses are the widest, and zygomatic arches the strongest, in the whole class Reptilia ; the lower jaw presents a like excessive development of the coronoid processes. These developments, for great size and power of action of the biting and grinding muscles, relate to a most extraordinary form and size of the teeth, which resemble paving-stones, and were evidently adapted to crack and bruise shells and crusts of marine Invertebrate.
The teeth of the upper jaw consist of an external or maxillary series, and an internal or palatal series. The maxillary series are supported in a marginal row of alveoli by the premaxillary and maxillary bones ; the palatal series are implanted in the palatine and pterygoid bones. The maxillo premaxillary teeth are five in number on each side, two implanted in the premaxillary, and three in the maxillary. The premaxillary teeth are subequal, smaller than the maxillary teeth ; their crowns are subhemispheric in P. laticeps, but in P. Andriani they present a bent, pointed, prehensile character. In P. laticeps the first maxillary tooth has a full oval crown, 41- lines by 4 in diameter ; the second measures 51- lines by 4.1 lines in diameter ; the third is subcircular, 8 lines in diameter, on the right side. The palatal series begins on the inner side of and consists of two teeth on each side. The first tooth has a full elliptical crown, 10 lines by 8 ; the second tooth, developed in the broad pterygoid bone, presents a full oval shape, 1 inch 9 lines by 1 inch 3 lines in diameter. In Placodus gigas and P. Andriani the palatal teeth, three in number on each side, are all of large size, slightly increasing from before backwards ; they are situated close together, forming on each side a series a little curved with the convexity outwards, and the interspace between the two series is very narrow. The first tooth is triangular, the second and third
are quadrangular ; each with the angles rounded, and the transverse diameter exceeding the fore and aft or longitudinal one. The maxillary teeth are much smaller than the palatal ones, have a rounded or subquadrate crown, are four in num ber, and of subequal dimensions. The premaxillary teeth, three in number on each side, are more remote and distinct from the maxillary teeth than in Placodus rostratus and P. laticeps; their crowns are more elongated and conical than in P. laticeps; the prehensile power of the prolonged premaxil lary part of the jaw being obviously greater in Placodus gigas than in P. laticeps or P. rostratus. The size of the last tooth in P. laticeps surpasses that of any of the teeth in the previ =sly discovered species. In proportion to the entire skull, it is the largest grinding tooth in the animal kingdom, the elephant itself not excepted.
All these teeth are implanted by short simple bases in distinct hollow sockets, subject to the same law of displace ment and succession as in other reptiles. By some it may be deemed requisite to separate generically the Placodi with two teeth from those with three teeth in each palatal series ; but the Placodus rostratus offers a transitional condition in the small relative size of the first two palatal teeth, and in the rounded form of all the teeth, from the P. Andriani to the P. laticeps.
We cannot contemplate the extreme and peculiar modifi cation of form of the teeth in the genus Placodus without a recognition of their adaptation to the pounding and crushing of hard substances, and a suspicion that the association of the fossils with shell-clad Mollusks in such multitudes as to have suggested special denominations to the strata containing Placodus (e. g., muschelkalk, terebratulitenkalk, etc.), is indi cative of the class whence the Plaeodi derived their chief subsistence.