BETWEEN TYPICAL REPTILES AND OTHER ANIMALS. 175 



skull, and it divides the pterygoid from the palatine bones. The 

 nares do not open upon the palate. 



The dorsal and caudal vertebrae of the Menopome resemble 

 the Crocodile's in having the ribs supported on transverse pro- 

 cesses ; and in some types tlie articulation of the rib's head is di- 

 vided. The ribs never encircle the viscera ; and there are never 

 neural spines. In the tail the chevron bones are anchylosed to 

 the centrum. The centrum appears to be biconcave. 



The scapula is very like that of the Crocodile, but widens at 

 the acetabulum for the humerus, so as to become j]-shaped. In 

 the Menopome epiphyses to the limb-bones are not ossified. 



The humerus is twisted, and expands widely at the distal 

 end. At the proximal end the radial crest is greatly developed, 

 but, from the twist in the bone, does not make an angle with the 

 shaft. 



The ulna and radius, though stouter in the Menopome, have 

 sufficient resemblance to make a detailed comparison necessary 

 with both Crocodilia and Testudinata. 



The carpus in the Menopome is unossified, and so far resem- 

 bles the condition of the Crocodilian distal carpal series, though 

 in other Urodelans all the elements are changed to bone. The 

 metacarpals and phalanges are compressed from above downward, 

 like those of some Dolphins. 



In the pelvis there is no near resemblance ; and the hind limbs 

 are formed more on the Lacertian than on the Crocodilian 

 plan. 



PAET TI. 



THE SIMILITUDES OF CHELONIAN BONES. 



§ 1. The Mammalian Characters of Chelonians. 



There is in Chelonians a nearer resemblance than in Croco- 

 diles to the usual plan of the mammalian posterior nares, since 

 they are divided by the vomer, and have their anterior lateral 

 border made by the palatine. And in mammals the anterior 

 nares are similarly single at their termination, except in the For- 



