70 ICHT HTOPTERT GIA . 



usually only four longitudinal rows of phalangeals in this limb, and 

 the radius (fig. 25) nearly square, with its anterior border notched. 

 Occasionally a small vacuity between radius and ulna. In some 

 cases pectoral limb scarcely larger than pelvic, and both of great 

 relative length. Neural spines of dorsal vertebrse more or less low 

 and wide. 



The cranial rostrum is frequently much elongated. The femur 

 (fig. 26) is nearly symmetrical, long and narrow, without a distinct 

 trochanteric ridge, and with considerable expansion of the distal 

 extremity. In the humerus the proximal extremity may either 



Fig. 26. 



Ichthyosaurus (cf.) tenuirostris. — Dorsal and distal aspects of the left femur ; 

 from the Lias of Street, t, tibial,/, fibular facet. 



have an oblong contour with the longer axis placed antero-poste- 

 riorly (i. e. parallel to the longer axis of the distal surface), or a 

 sub triangular contour, with the longer axis transverse. Its distal 

 extremity is usually much expanded. 



This group is unknown above the Lias, and its more generalized 

 species connect the Latipinnate group with Mixosaurus. The affi- 

 nities with the latter are shown by the contour of the radius and 

 tibia, and the occasional presence of a small vacuity between the 

 radius and ulna. 



Were it not for the difficulty of explaining the presence of the 

 same feature in the carpals and tarsals, the notch in the anterior 

 border of the radiale, tibiale, and phalangeals might be regarded as 

 the last trace of the shaft of a " long bone " ; and it is by no means 

 certain that this interpretation mav not really prove to be the true one. 



