ON BRITISH FOSSIL REPTILES. 147 



process in the Rhynchosaur is broadest behind, and commences there by two 

 roots or ridges, one from the upper and back part of each posterior articular 

 process : they meet at the posterior part of the summit of the neural arch, 

 whence the spinous process is continued upwards as a simple plate of bone, 

 its base extending forwards along about two-thirds of the length of the plat- 

 form, which then again divides into two ridges, which diverge from each other 

 in slight curves to the anterior and external angles of the neurapophyses. The 

 interspace of the diverging anterior crura of the base of the spine is occupied 

 by a triangular fossa, not continued into the substance of the spine ; this 

 fossa is bounded below by u horizontal plate of bone extended over the an- 

 terior part of the spinal canal, and terminated by a convex outline. 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 degi'ee that of the vertebrae of the Palaosaurus from the Bristol 

 conglomerate. The following are dimensions of the most perfect of the dorsal 



vertebrae of the Rhvnchosaurus : — t • 



•^ Lines, 



The length of the centrum 5^ 



Height of the articular end 3 



Breadth of the articular end 2f ^ 



From the lower margin of the posterior extremity of the centrum to 1 _ 



the posterior part of the base of the spine j 



From the lower margin of the posterior extremity of the centrum to 1 „ 



the summit of the spine j 



Antero-posterior extent of base of spine 4 



Breadth of the neural arch, from the outer margin of one anterior! _i 



articular process to that of the opposite side J 2 



Breadth of the neural arch at the interspace between the anterior and 1 . 



posterior oblique processes J 



Breadth of the neural arch across the middle of the spinal platform . 2 



Skull. — The most complete specimen yet obtained of this instructive part 

 of the skeleton of the Rhynchosaurus, is imbedded in a portion of the coarse- 

 grained sandstone from the Grinsill quarries. The lower jaw is in its natural 

 position, as when the mouth is shut, showing that the parts had not been dis- 

 located from the time of the death of the animal to its becoming imbedded in 

 the sand. 



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

 and with the upper facet arching down in a graceful curve to the apex, which 

 is formed by the termination of the muzzle. The very narrow cranial box; 

 the wide temporal fossae on each side, bounded posteriorly by the parietal and 

 the mastoid bones, and laterally by strong compressed zygomata ; the long 

 tympanic pedicle, descending freely and vertically from the point of union of 

 the posterior transverse and zygomatic arches, and terminating in a convex 

 pulley for the articular concavity of the lower jaw ; the large and complete 

 orbits ; and the short, compressed, and bent down maxillae, — all combine to 

 prove the i'ossil to belong to the Lacertian division of the Saurian order. The 

 mode of articulation of the skull with the spine cannot be determined in the 

 present specimen, but the lateral compression and the depth of the skull, the 

 great vertical breadth of the superior maxillary bone, the smaller relative size 

 of the temporal spaces, the great vertical breadth of the lower jaw, all prove 

 that it does not belong to a reptile of the Batrachian order. The shortness 



l2 



