SAUR'OPTERYGIA 231 



laries. In Xothosaurus the pterygoids extend back, under- 

 lapping the basi-sphenoid, as far as the basi-occipital, the 

 median suture uniting them being well marked to their ter- 

 mination ; and there is no appearance of vacuities like the 

 pterygo-sphenoid ones in Plesio- and Pistosaurus. The tym- 

 panies are relatively longer, and extend farther back in Pisto- 

 than in Plesiosaurus. There is no trace of lacrymals in 

 Pistosaurus ; and its maxillaries are relatively larger than in 

 Plesiosaurus. In Pistosaurus there are 18 teeth on each the 

 upper jaw, including the 5 premaxillary 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 trans- 

 verse section : the anterior teeth are proportionally larger than 

 the posterior ones than they are in Plesiosaurus. The dis- 

 proportion is still greater in Xothosaurus, in some species of 

 which the teeth behind the premaxillary and symphysial 

 terminal expansions of the jaws suddenly become — e.g., in 

 Xothosaurus mirabilis (fig. 68) — very small, and form a straight, 

 numerous, and close-set single series along the maxillary and 

 corresponding part of the mandibular bone. 



Both Xothosaurus and Pistosaurus had many neck-verte- 

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

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

 from the centrum to the neurapophysis ; but the surface, when 

 divided between the two elements, projected further outwards 

 than in most Plesiosauri. 



In both Xotho- and Pistosaurus the pelvic vertebra 

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

 relatively larger, vertically longer size, standing well out, and 

 from near the fore part of the side of the vertebra. 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 leu show thai tin 1 



