294 THE VERTEBRATE SKELETON. 



(fig. 35) there are only three digits. The North American Dino- 

 saurs present a continuous series ranging from a pentedactylate 

 plantigrade form like Morosaurus, to such a form as Hallopus 

 with a highly digitigrade and specialised pes reduced to three 

 functional digits, and a vestigial fifth metatarsal. The second, 

 third and fourth metatarsals in this form are nearly two- 

 thirds as long as the femur, and the calcaneum is drawn out 

 into a heel much as it is in most mammals. 



In Lacertilia, Orthopoda and many Chelonia, the ankle 

 joint comes to lie between the proximal and distal row of 

 tarsals as in birds. 



