268 FOSSIL REPTILES. 



convexity, and receives the bones of the fore-arm without pre- 

 senting to them two distinct facets. There are always two 

 bones in the fore-arm, but of httle mobihty. They are so 

 placed that the cubitus forms the external edge of the arm, 

 and the radius the internal. The upper head of the radius is 

 semi-circular, and a little concave ; the body is slender ; the 

 lower head compressed, and cut obliquely, so that it is shorter 

 on the cubital side. 



The cubitus is compressed ; its upper head is triangular 

 and cut obliquely. The lower head is squared. 



The pelvis of the tortoises is always composed of three bones, 

 contributing, as in quadrupeds, to the composition of the coty- 

 loid foss : these are an ilium, a pubis, and an ischion. At 

 the place where they unite to form the cotyloid cavity, each 

 bone has three faces, one for each of the two others, and one 

 for the cavity. 



The femur might easily be taken for the humerus of a qua- 

 druped ; its oval head is removed from the body of the bone 

 without being separated precisely by a narrow neck. Instead 

 of a trochanter, there is a transverse crest not much raised, and 

 separated from the head by a semicircular depression. The 

 middle of the bone is slender and round, and the bottom 

 compressed from front to rear, and widening by degrees to 

 form the lower head, which is a transverse portion of a cylinder, 

 a little inflected behind. 



The two bones of the leg are nearly straight. The tibia is 

 bulky and semicircular in the top, and again grows rather 

 bulky at bottom. The peroneum is more compressed, and 

 wider in the lower part. 



In all the parts now described there are slight variations ac- 

 cording to the subgenera, which, if necessary, shall be noticed 

 in our comparison of the fossils. 



In the sea-tortoise all the bones of the carpus are flat and 

 cut nearly square. The metacarpian of the thumb is short and 

 broad, the others are long and slender. There are but two 



