THE SKELETON OF REPTILES 41 



erect-walking reptiles ancestral to the mammals, this process was 

 directed forward, as in birds and mammals. The crocodilia, 

 unlike all other known reptiles, have the pubes excluded from the 

 acetabulum, and they do not meet in a median symphysis. This 

 character alone will distinguish any crocodilian from all other 

 reptiles. But there is some doubt as to the homology of the bones 

 usually called pubes in the crocodiles. Some of the bipedal 

 dinosaurs have the pubis forked, the anterior part directed down- 

 ward and forward, and not meeting its mate in a symphysis, the 

 posterior process long and slender, lying below the long ischium, 

 as in birds. Indeed, when this peculiarity of the dinosaurian 

 pubis was first discovered, it was thought to be an evidence of the 

 immediate relationship of birds; its structure is now interpreted 

 differently. 



POSTERIOR EXTREMITY 



The thigh bone or femur in reptiles, like the humerus, is variable 

 in size and shape. Only in those reptiles that walked erect is the 

 articulation of the head set off from the shaft of the bone by a 

 distinct neck. In others the articulation is at the extreme top of 

 the bone, since the thigh bones are habitually turned more or less 

 directly outward from the acetabulum and the long axis of the body. 

 The more or less pronounced rugosities at the upper end of the 

 femur, for the attachment of muscles, called trochanters, are not 

 easily distinguishable into the greater and lesser, as in mammals. 

 Sometimes, as in the erect-walking dinosaurs, there is a more or 

 less pronounced process on the shaft lower down, called the fourth 

 trochanter, for the attachment of caudal muscles. On the back 

 part of the shaft there is a ridge or line for the attachment of 

 muscles, corresponding to the linea aspera of the mammalian femur. 

 The projections at the lower end on the sides are called condyles. 



The two bones of the leg, or shin, are usually shorter than the 

 thigh bones, though in running and leaping animals they may be 

 quite as long or even longer. That on the inner or big-toe side is 

 called the tibia, and articulates with the distal end of the femur, 

 but chiefly with its inner condyle. It has a more or less well- 

 developed crest in front above for the attachment of the extensor 

 muscles directly, since there never is a patella in reptiles, and only 



