CRYPTODONTIA 241 



and sends outwards from each angle of its base a broad tri- 

 angular process with a flat articular surface ; the two anterior 

 surfaces look directly upwards, the posterior ones downwards ; 

 the latter are continued backwards beyond the posterior ex- 

 tremity of the centrum ; the tubercle for the simple articula- 

 tion of the rib is situated immediately beneath the anterior 

 oblique process. So far the vertebras of the Rhynchosaurus, 

 always excepting their biconcave structure, resemble the 

 vertebrae of most recent lizards. In the modification next to 

 be noticed, they show one of the vertebral characters of the 

 Dinosaur ia. A broad obtuse ridge rises from the upper convex 

 surface of the posterior articular process and arches forwards 

 along the neural arch above the anterior articular process, and 

 gradually subsides anterior to its base : the upper part of this 

 arched angular ridge forms, with that of the opposite side, a 

 platform, from the middle line of which the spinous process is 

 developed. Nothing of this kind is present in existing lizards ; 

 the sides of the neural arch immediately converge from the 

 articular processes to the base of the spine, without the inter- 

 vention of an angular ridge formed by the sides of a raised 

 platform. The base of the spinous process is broadest behind, 

 and commences there by two roots or ridges, one from the 

 upper and back part of each posterior articular process. The 

 anterior margin of the spinous process is thin and trenchant ; 

 the height of the spine does not exceed the antero-posterior 

 diameter of its base ; it is obliquely rounded off. The spinal 

 canal sinks into the middle part of the centrum and rises to 

 the base of the spine, so that its vertical diameter is twice as 

 great at the middle as at the two extremities : this modification 

 resembles in a certain degree that of the vertebras of the 

 Pola'osaurus from the Bristol conglomerate. 



The skull presents the form of a four-sided pyramid, com- 

 pressed laterally, and with the upper facet arching down in a 

 graceful curve to the apex, which is formed by the termination 



R 



