CHELONIA 



219 



surrounding them and attached to them. All these bones com- 

 pose what is called the carapace, which forms a complete roof in 

 the more terrestrial types, more or less imperfect, with vacuities 

 between the bones in the marine forms. On the under side, in 

 addition to the clavicles and the interclavicle, there are three 

 pairs of enlarged ventral ribs that go to form the plastron, solid 

 and complete in land turtles, with openings in the water forms. 

 And in the land forms 

 the plastron is more or 

 less firmly united with 

 the carapace. 



In the skeleton con- 

 tained within the box 

 thus formed is the very 

 peculiar pectoral girdle, 

 composed of scapula 

 and coracoid, the scap- 

 ula so peculiar that 

 the controversy as to 

 its homologies is not 

 yet quite settled. Most 

 authors, until recently, 

 have believed that its peculiar shape (Fig. 113) is due to the 

 co-ossification of the procoracoid with the scapula instead of as 

 usual its loss or union with the true coracoid, so called. We are 

 now pretty sure that this is not true, since in reality there is no 

 such bone as the procoracoid, the bone so called being the real or 

 true coracoid; and because, in the second place, the long anterior 

 projection called the procoracoid is really only an outgrowth of the 

 scapula itself and not a fused, separate bone. Hence the bone is 

 properly called the scapula-proscapula, and not the scapula pro- 

 coracoid, as it usually has been. The coracoids are elongate and 

 flattened and without the usual supracoracoid foramen, so gener- 

 ally present in reptiles. The only other reptiles having a simi- 

 lar structure of the scapula are the plesiosaurs, and it has been 

 because of this apparent resemblance that some good paleontologists 



Fig. 113. — Toxochelys; coracoid and scapula 



