EXTINCT ORDERS OF REPTILES. 215 



and abounding in the Lias, Oolites, and Chalk, but especially 

 characteristic of the Lias. As itself forming the order, the 

 essential characters of Iclithyomiims are those above given. 

 In all the species of the genus, the head is of proportion- 



rig. 6(54. — Ichthyosaurus communis. Lias. 



ately gigantic size, not separated from the trunk by a dis- 

 tinct neck, and prolonged anteriorly into a huge snout, the 

 jaws carrying a formidable array of conical teeth. The 

 orbits are of immense size, and the small apertures of the 

 nostrils are situated close beside them. The vertebral 

 column is long, and there is a long series of ribs, extending 

 from the neck to the tail, but none of them united in front 

 with a sternum. The absence of a breast-bone, however, is 

 compensated for by the development of a number of trans- 

 verse curved bones, which strengthen the abdominal walls, 

 and each of which consists of a median section and of three or 

 more overlapping pieces on each side. The vertebral centra 

 are deeply amphicoelous, the transverse and articular pro- 

 cesses being rudimentary, while the neural arches have for 

 the most part only a cartilaginous connection with the bodies 

 of their respective vertebrae ; so that the latter are commonly 

 found in a fossil condition in a perfectly detached state. 

 Beneath the caudal vertebrae are placed V-shaped " chevron- 

 bones." 



The pectoral arch (fig. 565, a) consists of a T-shaped 

 interclavicle, with a clavicle, coracoid, and scapula on 

 each side, the coracoids being of large size. The pelvic 

 arch (fig. 565, b) — there being no sacrum — is not directly 

 connected with the spine ; but the pubic and ischial bones 

 unite by symphysis. Both the pectoral and pelvic limbs 

 are in the form of paddles, the former being placed just 

 behind the head, and being generally much larger than the 

 latter. Each paddle (fig. 565, a) is composed of numerous 



