SAUROPTERYQIA. 161 



genera, only single-headed iu the majority of the later forms. 

 About two vertebra form a passage to the trunk-region, where 

 all the ribs are single-headed and articulate with the transverse 

 processes of the neural arch. The feeble sacrum, apparently 

 without fusion of centra, comprises from two to four vertebrae. 

 The caudal vertebrae exhibit both single-headed ribs articulated 

 with the centra, and also small chevron bones. The limb-girdles 

 become more robust and the limbs themselves more paddle-like 

 as the various successive genera are evolved. At first (fig. 102 A) 

 the median symphysis of the coracoids is not much extended, 

 and the scapulse are normal, well-separated by a clavicular arch. 

 In later forms (figs. 102 B E) the scapulae tend to meet in the 

 middle line in front, while the clavicular arch degenerates ; and 

 the latter eventually becomes a mere vestige, as the scapulae 

 not only fuse together by a ventral extension but also meet the 

 much-lengthened coracoids again in a median bar (fig. 102 E). 

 An obturator foramen always persists in the pelvis, but the 

 ilium becomes displaced backwards, articulating only with the 

 ischium (fig. 106 G). The limbs are elongated and compara- 

 tively slender in the Triassic genera, with five digits, the 

 normal number of phalanges, and often modified ungual 

 phalanges; there is also an entepicondylar foramen in the 

 humerus. In later genera they are true paddles, with 

 shortened radius and ulna, tibia and fibula, and still only 

 five digits, but the latter elongated by the addition of super- 

 numerary phalanges and destitute of modified claws. The 

 humerus and femur are noteworthy as exhibiting large conical 

 epiphyses (fig. 106 J), much like those of frogs and certain Che- 

 Ionia. Between the pectoral and pelvic arches the abdominal 

 wall is strengthened by a system of ventral ribs, especially 

 well-developed in the post-Triassic genera. The skin appears to 

 have been destitute of armour, and one Liassic fossil suggests 

 that there was a small, vertically extended caudal fin. 



One small nodule is known from the Upper Lias of Whitby, 

 having the outward appearance of a group of embryonic Plesio- 

 saurs. It thus seems likely that some of these aquatic reptiles 

 will prove to have been viviparous ; although no conclusive 

 evidence on the subject has yet been obtained. 



w. 11 



