THE BOXES IK THE EXALTOSAUllTA. 823 



also dissimilar — Plesiosaurs having no auditory excavation of the 

 (juadrate and squamosal, or median backward prolongation of the 

 parietal and superoccipital. Most living chelonians differ from 

 Plesiosaurs in not having nasal bones. 



The vertebral column has but little in common in the two types, 

 either in the form of the centrum, the condition of its articiilar ends, 

 the form of the neural arch, the attachment of the dorsal ribs, or 

 the proportional lengths of the different regions ; so that in no 

 region of the column, either as a whole or in the separate ver- 

 tebrfe, is there any resemblance to Plesiosauncs, the chief differ- 

 ences being : — that in chelonians the cervical vertebrae are opis- 

 thocoelous, proccelous, and biconvex, but never biconcave ; that 

 the neural arch has no neural spine, no cervical ribs, and has the 

 zygapophyses long and directed outward ; while the under surface 

 of the centrum is compressed, whereas in Plesiosaurs it is wide ; 

 the dorsal vertebrae are elongated, with flat articular ends, and 

 have the rib articulating by a single head directly with the cen- 

 trum, either between two vertebrae or at the anterior end ; the 

 caudal vertebrae have no neural spine in chelonians, and are pro- 

 ccelian or opisthocoelian. 



The chelonian scapular arch has, at first sight, nothing in com- 

 mon with Plesiosmirus ; but if the interspace between the coracoid 

 and pi-ecoracoid were ossified in Clielone, two expanded coracoids 

 meeting mesially would be formed, having a great resemblance to 

 those of Flesiosaurus. And such a result might also be attained by 

 continuous ossification of the coracoids with the hyosternal ele- 

 ments. Next it would be necessary to draw the scapulae forward 

 to be in the same plane with the coracoids. These, in the same 

 way, might become continuously ossified with the clavicles ; and 

 the backward angle of the clavicle might be represented by the 

 outer backward process sometimes seen on the plesiosaurian sca- 

 pulae. These bones would then take up the interclavicle between 

 them inevitably in the position seen in JPIesiosaurus, with its pos- 

 terior angle internal to the coracoids, as it is internal to the 

 hyosternal bones in chelonians. 



The pelvic arch is more like that of Flesiosaurus. The ilium 

 is similar, except that it is curved and the sacral end is more ex- 

 panded. The pubis has a similar large expansion in Chelone, but 

 not a kidney-shape. In the Tortoise the pubis meets the ischium 

 in the symphysis as in Flesiosaurus, and the iscliium has a sub- 

 triangular form and is directed backward ; but it is not in tlie 



