hatcher: diplodocus (maesh) 49 



with the posterior and internal surface of the astragalus. This arrangement of the 

 articular surfaces between the tibia, fibula, and astragalus allows of considerable 

 movement antero-posteriorly, while at the same time prohibiting almost all lateral 

 movement. It forms an exceedingly strong ankle joint similar to that which ob- 

 tains in ungulate mammals, and was especially well adapted to resist the strains to 

 which the ankle would have been subjected in the perambulations of so massive an 

 animal. Figs. 18 and 19 represent respectively the proximal and distal extremities 

 of tibia and fibula of No. 94. External and front \dews of these bones are shown 

 in PI. XI., Figs. 1 and 2. 



Measurements of No. 94. 



Greatest length of tibia 1006 mm. 40 |- in. 



" breadth at proximal end 274 " 11 | " 



" " "distal " 195 " Sj's " 



" length of fibula 1050 " -43 f " 



" breadth at proximal end 213 " 9|" 



" " "distal " 155 " 7 J" 



The Pes. 

 The Tarsus. — The osseous portion of the tarsus in Diplodocus, as in the alhed 

 genus Brontosaurus, has been reduced to an astragalus. This is a very broad bone, 

 deep externally, but rather thin, flat, and contracted internally. Superiorly it covers 

 the entire distal end of the tibia, with which it articulates by a continuous articular 

 surface, which is high and slightly convex externally, but low and somewhat exca- 

 vated internally, in order to accommodate the inferiorly produced internal malleolus 

 of the tibia. The external surface is separated from the anterior by a rather pro- 

 nounced ridge, but the articular surfaces of the two faces are confluent. The anterior 

 surface of the astragalus presents a broad, smooth, regularly convex surface for artic- 

 ulation with the proximal ends of metatarsals I., II., III., and the inner proximal 

 portion of metatarsal IV. The external portion of the astragalus is high and deep 

 and presents a deeply excavated lateral surface with an expanded anterior and in- 

 ferior margin developed into a long, narrow, semi-circular articular surface, which is 

 opposed to the internal margin of the distal end of the fibula. Posteriorly the astrag- 

 alus is much constricted, and consists of a comparativel}'- narrow ridge regularly 

 concave vertically and convex laterally, and separating the large external lateral 

 cavity just mentioned from a similar, but smaller, internal lateral cavity. Inferiorly 

 the astragalus presents a broad, rugose, and regularly convex plantar surface. This 

 in life was evidently covered with thick cartilaginous pads which were directly op- 



