Page:Quarterly Journal of the Geological Society of London, vol. 35.djvu/200

150 their dentition, generic distinction from both Goniopholis and Petrosuchus.

The number of maxillary and mandibular specimens, of which three are figured in Pl. IX. figs. 4, 5 and 7, exemplified a degree of constancy in size which begat a conviction that such was a character of the species; and, diminutive as were the Reptilia in question, their characters were indisputably those of the order Crocodilia. One of them, by the size and shape of certain teeth, came nearer to Goniopholis; another, by the same characters, resembled Petrosuchus; but the differences were such as could not have been obliterated by growth or age.

Theriosuchus approaches, like Goniopholis, nearer to the type of the broad-faced Alligators in the proportion of the antorbital part of the skull (fig. 1, o, n); but the dentition is more modified than in any other known Crocodile, recent or extinct, and approaches nearer to that which characterizes the Theriodont order of Triassic Reptilia.

The premaxillary teeth, five in number in each bone, are small; the three middle ones subequal, the first and fifth smaller; the maxillary teeth are divisible into laniaries (fig. 3, l) and carnassials or trenchant molars (ib. m). The first maxillary tooth is small, the second and third gain quickly in size, the latter (fig. 5, a) assuming the character of a canine; the fourth tooth (ib. b, and fig. 6, b) is a still larger canine; the fifth (fig. 6, c) and sixth (d) decrease in size somewhat suddenly, but in length rather than breadth of crown, and terminate the series projecting from the convex part of the alveolar border of the maxillary; the tooth c or d may be said to terminate the laniary series. Beyond d the teeth lose length and slightly gain in breadth; the crown assumes a triangular, laterally compressed or lamellate form, and the enamel is traversed, on the outside, by fine but distinct lines (fig. 6, e).

Of these sectorial or carnassial molars, some of the detached specimens of maxillary bones (figs. 4 and 5) indicate as many as eight or nine. The broad base or root of each tooth is not inserted into a separate socket, but is lodged in a recess of the outer alveolar wall; moreover the partitions between these recesses, are low or partial, and the teeth appear to have been applied thereto, without being so completely confluent therewith as in the pleurodont mode of fixation of the teeth in certain lizards. Hence in some of the specimens of the maxillary bone the incisors and canines only are retained, being rooted each in its own complete socket, while the molars have fallen out, and their partially separated recesses are shown as in the figures cited.

In the lower jaw the foremost tooth is rather larger than those which interlock with the middle premaxillary or 'incisor' teeth above; but not any of the succeeding laniary teeth attain the size