SAUROPTERYGIA. 251 



In Pistosaurus there are 18 teeth on each side of the upper 

 jaw, including the 5 preinaxillary teeth ; in Plesiosaurus there 

 are from 30 to 40 teeth on each side. In Pistosaurus the 

 teeth are relatively larger, and present a more oval transverse 

 section : the anterior teeth are proportionally larger than the 

 posterior ones than they are in Plesiosaurus. The dispropor- 

 tion is still greater in Nothosaurus, in some species of which (N. 

 aduncidens, v. m.) a pair of curved tusks recalls the peculiar 

 armature of Dicynodon ; whilst the teeth behind the premax- 

 illary and symphysial terminal expansions of the jaws suddenly 

 become — e.g., in Nothosaurus mirabilis (fig. 90) — very small, 

 and form a straight, numerous, and close-set single series along 

 the maxillary and corresponding part of the mandibular bone. 



Both Nothosaurus and Pistosaurus had many neck-verte- 

 brae, and the transition from these to the dorsal series was 

 effected, as in Plesiosaurus, by the ascent of the rib-surface 

 from the centrum to the neurapophysis. 



In both Notho- and Pistosaurus the pelvic vertebra 

 developes a combined process (par- and di-apophysis), but 

 relatively larger, vertically longer than in Plesiosaurus. This 

 process, with the coalesced riblet, indicates a stronger ilium, 

 and a firmer base of attachment of the hind limb to the trunk, 

 than in Plesiosaurus. Both this structure and the greater 

 length of the bones of the fore arm and leg, shew that the 

 muschelkalk predecessors of the liassic Plesiosauri were better 

 organized for occasional progression on dry land. 



A comparison of remains of Plesiosauri has shewn, that 

 specific distinctions are accompanied with well-marked dif- 

 ferences in the structure and proportions of answerable 

 vertebrae, but are not shewn in small differences of number 

 in the cervical, dorsal, or caudal vertebrae. When any region 

 of the vertebral column presents an unusual excess of develop- 

 ment in a genus, such region is more liable to variation, 

 within certain limits, than in genera where its proportions are 



