1903] Lower Jaw OF Dryptosaurus INCRASSATUS. 135 



The surangular is broadly arched above, as seen in side view, 

 and almost completes the remainder of the outer surface of the 

 mandible, the posterior end of the angular being- visible inferiorly 

 to a limited extent. The surangular is strengthened exteriorly, 

 near its upper border, by a prominent rounded ridg-e extending 

 for some distance forward from the articular cotylus into the 

 composition of which this bone enters. It embraces the articular 

 anteriorly and passing- beneath it extends as far back as the 

 posterior limit of that element. It is pierced by a large foraminal 

 opening at about one-fourth its length in advance of its back 

 termination and at about its mid-depth ; its inner surface in this 

 region is deeply concave (figure 4). Below the foramen the bone 

 becomes gradually thinner, where it is overlapped by the dentary, 

 and is continued forward with a thickness inferiorly of only a few 

 millimetres, although posteriorly and along its upper border it is 

 a strong and robust bone. 



The articular is small and compact, roughly triangular in 

 shape, and is scarcely seen except when viewed from above. It 

 forms about two-thirds of the cotylus and is overlapped on its 

 inner side by the angular, which extends nearly as far back as 

 either the surangular or the articular. Its breadth exceeds its 

 antero-posterior diameter. 



The cotylus is transverse, strongly bifossate and evidently 

 points to a strictly upward and downward motion of the jaw, as 

 the distal end of the quadrate fits closely into it. The movement 

 of the jaw is, therefore, apparently restricted, and differs from 

 that of Sphenodon in which the articulating surface is nearly four 

 times as great antero-posteriorly as the condyle of the quadrate 

 and admitted of a backward motion of the mandible. 



The slender bone meeting the surangular below the articular, 

 and embracing the latter element on its inner surface, is regarded 

 as the angular. It passes forward on the inner surface of the 

 ramus in contact with the inferior edge of the posterior extension 

 of the dentary but is, unfortunately, broken in both rami of the 

 1884 specimen at a point slightly behind the mid-length of the 

 surangular. The break in both halves of the jaw at this point is 

 unfortunate as it is here that the junction of the angular with the 

 splenial would have been looked for. It is probable, however, that 



