No doubt the most numerous examples of similarly-shaped teeth for a like purpose are afforded by the class of fishes, as, e. g., by the extinct Pycnodonts, and by the wolf-fish (Anar rh,ichas lupus) and the Cestracion of the existing seas. But the reptilian class is not without its instances at the present day of teeth shaped like paving-stones, of which certain Australian lizards exhibit this peculiarity in so marked a degree that the generic name Cyclodus has been invented to express that peculiarity. Amongst extinct reptiles, also, a species of lizard from the tertiary deposits of the Limagne in France presents round obtuse teeth, of which the last, in the lower jaw, is suddenly and considerably- larger than the rest.
Nothosauras, Simosaurus, and Pistosaurus present the same evidences of lacertian affinities in the division of the nostrils by the median extension of the premaxillary backwards to the nasals, the same thecodont dentition, and the same circum scription of the orbits and temporal foss as in Placodus there is also a general family likeness in the upward aspect of these apertures, accompanying an extreme depression of the skull. The muzzle, though varying greatly in length in these genera, presents the same obtuseness ; and the alveolar border of the jaws the same smooth outward convexity which we observe in the Placodus. The peculiar confluence of the elements of the upper and lower zygomatic arches,—i. e., of post-frontal and malar,—forming the broad wall of bone behind the orbit, is continued still farther backwards in the Simosaurus. In Pistosaurus the elongated post-frontal, malar, and aqua mosal are united together in one deep zygomatic arch, which has the mastoid and tympanic for its hinder abut ment.
It is remarkable that hitherto no vertebra; or other bones of the trunk or limbs have been found so associated with the teeth of Placodus, as to have suggested their belonging to the same species. Usually, after the indication of a reptile by detached teeth, the next step in its reconstruction is based upon detached vertebrae. The twelve or more evidences of
Placodus, afforded by bone as well as tooth, are all portions of the skull. It is possible that some of the singularly modified vertebrae from the muschelkalk, next to be described, may belong to the Placodus ; and the same surmise suggests itself in reference to some of the limb-bones from the muschelkalk that cannot be assigned to other known saurian genera.
The obvious adaptation of the dentition of Placodus to the crushing of very hard kinds of food, its close analogy to the dentition of certain fishes known to subsist by breaking the shells of whelks and other shell-clad Mollusks, and the cha racteristic abundance of fossil shells in the strata to which the remains of Placodus are peculiar, concur in producing the belief that the species of this genus were reptiles frequenting the sea-shore, and probably good swimmers. But as at present we have got no further than the head and teeth in the recon struction of this mezozoic form of molluscivorous reptile, the present notice will conclude with a remark suggested by the disposition and form of the teeth. In all the species, under the rather wide range of specific varieties of the dentition, there are two rows of the crushing teeth in the upper jaw, and only one row in the lower jaw, on each side of the mouth ; and the lower row plays upon both upper rows, with its strongest (middle) line of force directed against their inter space. Thus the crushing force below presses upon a part between the two planes or points of resistance above, on the same principle on which we break a stick across the knee ; only here the fulcrum is at the intermediate point, the moving powers at the two parts grasped by the hands. It is obvious that a portion of shell pressed between two opposite flat sur faces might resist the strongest bite, but subjected to alternate points of pressure its fracture would be facilitated.*