3 22 PUOF. 11. G. SEELEY ON SIMILITUDES OF 



saurs ; the zygapopliyses are more projecting; aud tlie transverse 

 process of the dorsal vertebrse is lamellar and not cylindrical. 

 The caudal vertebrse, besides differing from Plesiosaurus in their 

 length, differ in the lengths of the chevron bones, which are not 

 attached each between two vertebrae in crocodiles, but to the basal 

 hinder articular margin of its own vertebra. The short cervical 

 vertebrse of Plesiosaurs have a hatchet or jj-shape, similar to that 

 seen in crocodiles ; and the articular head is sometimes divided 

 to articulate with two facets on the centrum, but never so 

 deeply divided as in the crocodile, where the upper liead ar- 

 ticulates with a tubercle on the neural arch, while the lower head 

 remains on the centrum, in all the pectoral vertebrae and all the 

 cervicals after the first two. 



The dorsal ribs differ from those of Plesioscmriis in their com- 

 pression from above downward, and in the articular end uniting 

 with the transverse process by two heads. 



The pectoral girdle is not similar ; for in the crocodile the cora- 

 coids are divided by the sternum, and they have but little antero- 

 posterior extension, corresponding only with the thick anterior part 

 of the coracoid in Plesiosaurus, and the scapulae are directed towards 

 the back ilistead of converging forward in the same plane with the 

 coracoids. JVor are the pelvic girdles similar. For the ilium of 

 the crocodile has a vertical expansion very unlike the subcylin- 

 drical tapering form seen in Plesiosaurs. The backward direc- 

 tion and symphysial elongation of the ischium is similar ; but the 

 small elongated triangular pubis of the crocodile is very unlike 

 the broad reniform bone convex in front seen in Plesiasaurus. 



The limb-bones cf the tw^o groups have no character in 

 common. 



§ 4. The Chelonian Characters of Plesiosaurus. 



There is considerable resemblance of form between the out- 

 lines of the skxills of some Plesiosaurs and some chelonians. And 

 in both the temporal fossae are large(except in marine Chelo- 

 nia), and only divided from the circular orbits by a narrow post- 

 frontal bone. The orbits of chelonians are more vertical, and 

 are w^anting in superorbital and lachrymal bones. The anterior 

 uares are near to the orbit ; but they have a single termination, 

 and the skull has no extension anterior to them ; so that all the 

 preorbital part of the chelonian skull is unlike, and not compa- 

 rable with the plesiosaurian ; the part behind the temporal fossa is 



