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S. W. WILLISTON 



rear themselves on the hind legs in running. In such locomotion 

 the leg is brought more nearly at right angles with the plane of the 

 plantigrade foot, and the result has been a closer union of the 

 proximal bones of the tarsus, either with each other or with the 

 leg bones. The earliest known reptile, from the Coal Measures of 

 Ohio, had only two bones in the proximal row, the astragalus and 

 calcaneum, and, even as early as the beginning of Permian times, 

 the centralia and fifth tarsale had begun 

 to disappear. In the elevation of the heel 

 from the ground a further union of the 

 proximal bones took place in some forms, 

 Pareiasaurus, for instance, and some living 

 lizards and turtles. In modern reptiles 

 the foot is plantigrade. Among the first 

 results of digitigradism was the closer 

 union of the astragalus with the tibia, as 

 seen in Ornithomimus and other bipedal 

 dinosaurs, becoming more intimate in a 

 more or less sutural union in Rhampho- 

 rhynchus, and the sutural obliteration in 

 Pteranodon and birds, and the final union 

 of the tarsalia with the metatarsals in the 

 latter. In known reptiles the chief joint 

 between the leg and foot is intratarsal, 

 that is between the first and third rows. 

 In the mammals it is between the tibia and the astragalus. One 

 wonders how the change occurred in the ancestral reptiles. 



Not only has the vertical posture of the leg been the cause of 

 the loss of tarsal bones, and of the change of the chief tarsal joint to 

 the immediate end of the tibia, as in mammals, or its functional 

 equivalent in the dinosaurs, ptderodactyls, and birds, but it has 

 also been the cause, I believe, of the reduction of the phalanges in 

 turtles, theriodonts, and mammals. In the most rectigrade 

 posture of the lower leg and foot of all turtles, the land tortoises, 

 but two phalanges remain in each toe. In the rectigrade Sauropoda 

 no toe has, I believe, more than four phalanges; and the tarsal 

 bones are much reduced. 



Fig. 3. — Trimerorhachis , 

 part of skull, from behind: 

 q, quadrate; qf, quadrate 

 foramen; art, articular; 

 s, squamosal; qj, quadrato- 

 jugal. One-half natural 

 size. 



