Ichthyosauria. 



37 



The humerus and femur are relatively short, hue the radius Wall-cases, 

 and tibia are still shorter, and may be reduced to oblong bones No. 14. 

 in which the breadth is greater than the length. The humerus Table-cases, 

 has no foramen. Usually the anterior pair of (pectoral) paddles ^°^' ' ' 

 is the larger (see Figs. 45 A, B; and 46 A, B). The humerus 

 and femur in this order are unique in that, instead of having 

 convex condyles for the articulation of the fore-arm (I'adius and 

 ulna) they present distinct concavities for their reception. 



Fig. 46.— (a) Dorsal aspect of the left pectoral, and (b) ventral aspect of the right pelvic 

 limb of rchthj/osauriis mttrmallus (Conybeare) ; Lower Lias, Lyme Regis, Dorsetshire. 

 A. h, humerus; r, radius; «, ulna ; r 1, radiale ; i, intermedium; u\, ulnare; c 1, 

 c2, centralia; B./, femur; .t, tibia; ./, fibula; il, tibiale; /I, flbulare; i, inter- 

 medium. 



These old marine lizards mixst have exercised the same 

 repressive action over the teeming animal population of the old 

 Liassic seas that the sharks do iu our seas at the pi'esent day. 

 They existed during the long period of geological time repre- 

 sented by the several formations extending from the Upper 

 Trias and Rhastic to the Chalk inclusive {see Table of Stratified 

 Rocks, p. x.) , but they occur in the greatest abundance, both as 



