FOSSIL REPTILES. 267 



wide. The acromial branch is compressed. The angle which 

 it makes with the omoplate is marked, but less so than in the 

 marine tortoises. 



The chelydes have the coracoid bone wider and shorter 

 than the fresh-water tortoises, but less so than in the land- 

 tortoises. 



In the trionyx, the angle is sufficiently marked, but the 

 coracoid bone is distinguished by a peculiar form. It is wider 

 than in the other sub-genera. Its external edge is convex, 

 and is continuous with the hinder edge, while the internal is a 

 little concave. 



The humerus of the tortoise must turn singularly on its axis, 

 to place the fore-part in the position which the osseous cuirass 

 requires, which leaves it no passage but by a narrow emargina- 

 tion. The head proceeds more out of the axis than in any other 

 animal, and that towards the upper face. It is a segment of a 

 sphere, and very concave. The two tuberosities are very large, 

 very salient, and leave between them a concavity, the same as 

 exists between the condyles of the humerus in the majority of 

 the mammifera. The internal tuberosity is the largest. It 

 has the form of a long obtuse crest, analogous to the deltoid 

 crest, and which receives the same muscles. The other tube- 

 rosity also forms a crest, but much shorter. The body of the 

 bone itself is arched ; and its concavity which, in man, would 

 be anterior, is here, in general, lower. The opposite face is 

 convex. In the upper part is a small hollow, opposite the end 

 of the foss, which is between the two tuberosities. 



The bottom of the bone is widened, and a little flatted from 

 front to rear. On its external edge may be remarked a fur- 

 row, not very distinct in the land-tortoises, deeper in the 

 emydes, the chelydes, and the trionyx, and which, in the 

 marine tortoises, separates the lower head of the bone into two 

 unequal parts. This furrow is the best character for distin- 

 guishing the lower part of the humerus from that of the femur. 

 This lower head is transversely oblong, and of an uniform 



