394 



ICHTHYOSAURIA. 



The atlas and axis vertebrae (G) are usually fused together and possess 

 three wedgelike hypocentra, one in front of the atlas, the next between 

 the atlas and axis, and the third behind the axis. Chevron bones are 

 found on the caudal vertebrae but their halves generally remain separate. 

 The hind end of the vertebral column is bent ventralwards into the ventral 

 lobe of the caudal fin. Abdominal ribs are present consisting of a median 

 piece and two or three lateral pieces on each side (Fig. 218 H). There is 

 no bony sternum. The shoulder girdle is lizard-like and very strong ; 

 it is shown in Fig. 218 M. The pelvic girdle is feeble (Fig. 218 N) ; all the 

 bones join in the a-cetabulum, but the ilia are not attached to the vertebral 

 column and it is doubtful if there was a pubic or ischiadic symphysis. 



The skeleton of the limbs is highly peculiar (Fig. 219). The long bones 

 (humerus. femur, radius, ulna, tibia, fibula, metacarpals) are all much 



shortened, sometimes broader than 

 long. Moreover the phalanges are 

 very numerous and the digits some- 

 times appear to be more than five 

 (to eight or nine). This is caused 

 partly by bifurcation of some of the 

 digits and partly by the presence 

 of an additional row of phalanges 

 on the preaxial and postaxial side 

 of the manus and pes(m.r,m.u,m.tb). 

 The carpalia of the distal row are 

 three or five, and the number of 

 digits is said to vary from three to 

 five, but this is not allowing for 

 the extra rows of phalanges above 

 referred to. It is important to 

 notice that the relatively large size 

 of the manus and pes (as compared 

 with the rest of the limb) is obtained 

 not by an increase in length of the 

 phalanges but by an increase in 

 their number. This is a peculiarity 

 of the Ichthyosauria which is met 

 with to a very small extent if at all 

 in other reptiles or in mammals. 

 The apparent increase in the num- 

 ber of the digits as indicated by the 



number of rows of phalanges is a most remarkable feature met Math in no 

 other pentadactyle form. Whether it is to be regarded as a polydactyle 

 condition such as must have existed in forms connecting the piscine with 

 the pentadactyle type is uncertain ; no remains of animals with such inter- 

 mediate types of limbs being known to us. The skin appears to have 

 been leathery and without scales or dermal plates. 



' That the Ichthyosauria were viviparous appears to be indicated by 

 the fact that specimens of them are found containing the remains of fair 

 sized young individuals in what must have been the abdominal cavity. 

 Their coprolites are known and they contain scales and bones of fishes and 

 fragments of Cephalopoda. The coprolites also show indications of having 

 passed through an intestine with a spiral valve. 



There is very little variation in structure in the species known to us. 



FIG. 219. A anterior, B posterior limb 

 of Ichthyosaurus intermedms (after 

 Huxley). Cp. carpalia ; E femur ; 

 / fibulare ; Fb fibula ; H humerus ; 

 i intermedium ; Me metacarpals ; 

 m.r extra radial digit ; Mt metatarals ; 

 m.tb extra tibial digit ; m.u extra ulnar 

 digit ; Ph phalanges ; R radius ; 

 T tibia ; Uibiale ; Ts tarsalia ; U ulna ; 

 1, 2, 3, 4, 8 digits. 



