358 PROFESSOR OWEN, ON THE RHYNCHOSAURUS. 



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 vertebra? of the Palceosmirus from the 

 Bristol conglomerate*. The following are dimensions of the most perfect 

 of the dorsal vertebra? of the Rhynchosaurus : — 



Lines. 



The length of the centrum 5-£- 



Height of the articular end 3 



Breadth of the articular end 2-f- ^ 



From the lower margin of the posterior extremity of the cen- 

 trum to the posterior part of the base of the spine 5 



From the lower margin of the posterior extremity of the cen- 

 trum to the summit of the spine 9 



Antero-posterior extent of base of spine 4 



Breadth of the neural arch, from the outer margin of one 



anterior articular process to that of the opposite side 8-J- 



Breadth of the neural arch at the interspace between the ante- 

 rior and posterior articular processes 4 



Breadth of the neural arch across the middle of the spinous 

 platform 2 



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

 part of the skeleton of the Rhynchomurus 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 dislocated when they became 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 cranium, the wide temporal fossa on each side, 

 bounded behind by the bifurcations of the parietal and the mastoid, and 

 laterally by a strong compressed zygoma, with a long tympanic pedicle 

 descending vertically from the point of union of the transverse and 



* Geological Transactions, Second Series, Vol. V. p. 349, pi. xxix. 



