1074 



CLASS REPTILIA. 



Family Plesiosaurid^e. — With this group we come to the con- 

 sideration of the typical members of the order, all of which were 

 adapted for a purely aquatic life, and probably frequented coasts 

 and estuaries. In the skull the pterygoids di- 

 verge posteriorly, and do not overlie the basi- 

 sphenoid ; while there were small infraorbital 

 vacuities in the palate. The dorsal vertebrae 

 have long transverse processes. In the pectoral 

 girdle the scapulas have large ventral plates, 

 which may meet in the middle line ; and the 

 symphysis of the coracoids is much elongated, 

 and extends in advance of the scapular articula- 

 tion. In the limbs (fig. 994) the humerus and 

 femur were comparatively short and distally ex- 

 panded ; the former being devoid of a foramen. 

 The bones of the second segment are likewise 

 very short, and strangely altered from the normal 

 form. In certain cases, moreover, a third bone 

 (fig. 998) articulates with the humerus and 

 femur, of which the homology will be discussed 

 under the head of the Ichthyopterygia. The 

 metacarpals and phalangeals are, however, still 

 elongated, but the number of the latter is in- 

 creased beyond the normal complement. Further, the terminal 

 claws have disappeared ; and the whole limbs were doubtless en- 

 veloped in a common integument, to form paddles after the fashion 

 of the turtles. The coracoid (fig. 988) is remarkable for its great 



Fig. 994. — Dorsal as- 

 pect of the left pectoral 

 limb of Plesiosaurus 

 Hawkinsi ; from the 

 Lower Lias of Dorset- 

 shire. Reduced, a, Hu- 

 merus ; b, Radius ; c, 

 Ulna. 



Fig. 995. — Skeleton of Plesiosaurus dolichodirus; from the Lower Lias. Greatly reduced. 

 (After Conybeare.) 



antero-posterior length, and has no fontanelle. This family includes 

 some forms of huge dimensions ; its range extending from the 

 Rhaetic, or Uppermost Trias, to the Chalk. 



The Plesiosauridce, like the Crocodiles of the present day, differ 

 greatly among themselves in the relative length of the mandibular 

 symphysis, as is shown in the accompanying woodcut. In the 

 present family it appears, however, that the result of evolution and 

 specialisation has been towards the gradual lengthening of this sym- 



