180 REPTILIA. 



in the adult, and they bear three subvertebral wedge-bones 

 (hypocentra or intercentra, w), the largest between the occipital 

 condyle (b.occ.) and the atlas (c 1 ), the next between the atlas 

 and axis (c 2 ), the smallest between this and the third centrum. 

 The two halves of the neural arch of the atlas (n 1 ) are separate ;. 

 each of the other neural arches of the vertebral column is in 

 a single piece, articulating with its neighbours by feeble zyga- 

 pophyses, and never fused with the centrum. There is no 

 sacrum, and the pelvic arch (fig. Ill N) is freely suspended 

 in the abdominal muscles. The anterior caudal vertebral 

 centra (fig. Ill J) bear a single pair of tubercles for the support 

 of single-headed ribs (?), which gradually disappear behind 

 when the vertebrae enter the caudal fin. The two halves of 

 the chevron bones (ch.) remain separate. The pectoral arch 

 (fig. Ill M) comprises a pair of robust coracoids (co.) meeting in 

 a long median symphysis, a pair of scapulas (sc.) without any 

 marked prescapular process, also slender clavicles (cl.) and a 

 T-shaped interclavicle (i.cl.). The humerus, radius, and ulna 

 in the typical genera are short and stout, and insignificant in 

 size compared with the remainder of the paddle, which com- 

 prises a variable number of irregular digits, each with very 

 numerous phalanges usually pressed together into an almost 

 inflexible mosaic. Each half of the freely suspended pelvis 

 consists of the usual three elements, which are all long and 

 slender and apparently meet in the acetabulum. The hind 

 limb is of the same character as the fore limb. Abdominal 

 ribs (fig. Ill H, abd.) are well-developed throughout the ventral 

 body- wall between the pectoral and pelvic arches ; but there is 

 no dermal armour. A median dorsal and a triangular caudal 

 fin are known to occur in one species of Ichthyosaurus as. 

 mentioned below. 



Some of the Ichthyopterygia at least must have been 

 viviparous, examples of Ichthyosaurus from the Upper Lias of 

 Wiirtemberg occasionally exhibiting as many as seven embry- 

 onic skeletons within the abdominal region. 



Mixosaurus. To this genus are referred most of the known frag- 

 mentary remains of Ichthyopterygia from the Triassic formations, and 

 nearly all of these are comparatively small. The teeth are fewer than 



