1182 



CLASS REPTILIA. 



Fig. 1081. — Left side of the pelvis of a young 

 Alligator. II, Ilium ; Is, Ischium ; P, Pubis; 

 a, b, b, Acetabulum, with its vacuity; F, 

 Obturator notch ; t, *, Cartilaginous pro- 

 cesses of the ischium and ilium. 



and subcorneal, or laterally compressed. In the sternal region the 

 sternum itself is cartilaginous ; and there is a bony interclavicle, but 



generally no clavicle. In the 

 pelvis (fig. 1 081) the ilium is 

 short and deep, without distinct 

 preacetabular or pubic processes ; 

 the ischium is stout, and devoid 

 of obturator process ; while the 

 pubis x is directed downwards 

 and forwards, and is frequently 

 excluded from the acetabulum. 

 In regard to the pectoral girdle 

 and limbs, it will suffice to say 

 that all the bones are solid ; 

 that the coracoid has a fonta- 

 nelle, and may be either short 

 or long ; that in the humerus 

 and femur the heads are im- 

 perfectly differentiated, that of 

 the latter being compressed and 

 placed very obliquely to the plane of the condyles ; while the shaft 

 of the femur has no inner trochanter. Moreover, the tibia has no 

 cnemial crest at its proximal extremity. The habits of all the 

 members of the order are quadrupedal; and the feet (fig. 1079) 

 are plantigrade, those of the hind limbs being partially webbed. 



The existing Crocodiles present many peculiarities in regard to 

 the soft parts ; but since we do not know whether the same features 

 occurred in the generalised fossil forms, and cannot compare them 

 with extinct orders, it is unnecessary to allude to them further. 



This order is peculiarly interesting, not only as containing the 

 on y existing members of the Archosaurian branch ; but also since 

 it affords a beautiful example of the gradual evolution of specialised 

 characters as we ascend in the geological scale. 



Suborder i. Aetosauria. — This provisional suborder includes 

 but a single family, which Dr Baur places in the Crocodilia, although 

 Professor Cope regards it as more nearly related to the Rhyncho- 

 cephalia, to which it perhaps belongs. 



Family Aetosaurid^e. — This family is typically represented by 

 the genus Aetosaurus, of the Upper Trias of Wiirtemberg ; a small 

 form with Crocodilian armour and limbs, but with the metatarsals 

 much elongated, and approaching in many points of its organisation 

 to the Theropodous Dinosauria, to which Professor Marsh regards 



1 It has been suggested that the bone termed pubis in the Eusuchia is really a 

 prepubis. 



