THE BONES IN THE ENALIOSAUIII.* . 299 



proximal end of the femur in Chelone, as to give the Porpoise- 

 humerus a character very unlike Ichthyosaurus. The ridges on 

 the humerus of Ornithorhynchus may also be compared. 



In no mammal is a parallel found to the shortness of the 

 ichthyosaurian ulna and radius, or to the uniform (commonly 

 quadrate) shape of the other bones of the limb or to their arrange- 

 ment, so that every surface except the exterior surface commonly 

 touches another bone in the adult. Some Ichthyosaurs have a 

 separate olecranon-ossification. 



The femur in shortness and strength recalls some Seals ; but 

 in arrangement of parts the resemblance is closest to Ornitho- 

 rhynchus, which similarly has lateral trochanters which extend the 

 width of the bone at the proximal end, though in Ichthyosaurus 

 they are not divided from the rounded articulation. In Ichthyo- 

 saurus the bone is shorter, compressed at right angles to the head 

 at the distal end, which does not articulate chiefly with the tibia, 

 but gives an equal flattened facet to both tibia and fibula. JS"o 

 mammal offers any parallel to the other bones of the hind limb, 

 though in Cetacea the limbs are similarly enclosed in a fin-like 

 sheath. 



Prof. Owen has thought that since in Cetacea the terminal 

 caudal vertebrae supporting a transverse fin are compressed from 

 above downwards, we may infer that Ichthyosaurus had a vertical 

 fin, since the terminal caudal vertebrae are compressed from side 

 to side. But in the human species the caudal vertebrae are com- 

 pressed from above downward, and in Crocodiles they are com- 

 pressed from side to side, without in either case carrying a corre- 

 sponding terminal fin. 



§ 2. The Avian Characters of Ichthyosaurus, 



Many birds, in general form of the head, resemble Ichthyosaurus 

 in its difl'erent species ; but in details the correspondence is not 

 close. Thus, though in both the (usual) backward position of 

 the external nares prolongs the prem axillary bones backward, 

 diverging, along the alveolar border, yet in birds a median ray 

 is prolonged backward between the nares, and overlapping the 

 large nasal bones, so as to nearly hide them, and look from the 

 outside as though it divided them ; while in Ichthyosaurus there 

 is no such median ray, and the separate premaxillary bones are 

 divided by nasal bones relatively larger than those of Struthious 

 birds. The premaxillary bones usually make more of the palate 



