ORDER III 



ICHTHYOSAURIA 



169 



the neural arch. The anterior caudals bear only a single pair of tubercles for 

 the support of the single-headed ribs, and these gradually disappear posteriorly 



Pig. 267. 



Ichthyosaurus trigonus, Owen. Kimmeridge Clay; Wooton- 

 Basset, England. Posterior dorsal vertebra, 1 / 3 . a, b, Tuber- 

 cular and capitular facettes for ribs (after Lydekker). 



Fig. 268. 



Ichthyosaurus, sp. Upper Lias; Banz. 

 Franconia. Caudal vertebra, l/j. 



(Fig. 268). The halves of the chevron bones usually remain separate, but the 

 neural arch of each vertebra except the atlas is undivided. A sharp down- 

 ward deflection of the column occurs in the posterior part of the tail, where 



Fig. 269. 



Ichthyosaurus guadricissus, Quenst. Upper Lias; Holzmaden, Wiirtemberg. Skeleton showing 

 outline of integument and dorsal and caudal tins, Vio (after E. Fraas). 



the vertebrae enter the base of the large, triangular caudal fin. The latter is 

 expanded in a vertical instead of horizontal plane, and must have been a very 

 powerful swimming organ. A median dorsal fin has also been observed in a 

 specimen from the Lias of Wiirtemberg (Fig. 269). 



Ribs are present on all the precaudal vertebrae, and reach their greatest 

 elongation between the tenth and thirteenth centrum, after which they 

 gradually decrease in size toward the pelvis. From here on they continue 

 single-headed, straight, and more like lateral processes than ribs, and become 

 obsolete toward the caudal fin. The long slender ribs of the trunk are 

 recurved, subcylindrical in section, and in most species longitudinally grooved. 

 A median longitudinal and one or two paired lateral series of splint-like 

 abdominal ribs are developed. 



The pectoral arch (Fig. 270) is unusually powerful, and indicative of 

 excellent swimming ability. There is no ossified sternum for the attachment 

 of ribs, but a T-shaped interclavicle is present in the median line, in part 

 overlapping the robust coracoids, whose inner edges meet in a long symphysis. 



