182 



REPTILIA. 



layer of cement. The neural arches of the vertebrae are much laterally 

 compressed, and there is usually only one median zygapophysial facette. 

 The humerus and femur exhibit only two distal facettes, in the first 

 case for radius and ulna, in the second for tibia and fibula. The carpal, 

 tarsal, metacarpal, and metatarsal bones are for the most part com- 

 pletely ossified in the adult and closely pressed together like an irregular 

 mosaic. The radiale (tibiale), intermedium, and ulnare (fibulare) are 

 always recognizable, but it seems impossible to determine the precise 



A. 



B. 



B 



" A 



FIG. 113. 



PECTORAL PADDLES OF ICHTHYOPTERYGIA. 



Ichthyosaurus conybeari; ventral aspect of left pectoral limb, about one- 

 half nat. size. L. Jurassic (L. Lias); Lyme .Regis. (After Lydekker.) 



Ophthalmosaurus iccnicus; ventral aspect of right pectoral limb, about one- 

 seventh nat. size. U. Jurassic (Oxford Clay) ; Peterborough. (From 

 spe.cimen in Leeds collection, British Museum, no. 85.) 

 h, humerus; /, intermedium; p, pisiform; r, radius; u, ulna. 



homologies of any of the more distal components of the liinb, or to 

 decide upon the primitive number of digits. The best-preserved skele- 

 tons hitherto discovered are from the European Lias, and they sometimes 

 display traces of the unarmourecl integument. One example of /. quad- 

 riscissus from the Upper Lias of Wurtemberg (fig. 112) exhibits remains 

 of a triangular dorsal fin and also a large, vertically extended caudal fin, 



