Vol. 56.] ARISTODESMUS RiiTlMEYERI. 639 



of the Saurischia, or indeed in any other Anomodonts. The distal 

 articulation appears to truncate the distal end of the bone somewhat 

 obliquely, in the manner seen in Pareiasaurus. It makes no 

 approach to the trochlear condition of the femur in Tribolodon and 

 other Theriodonts. The internal condyle of the femur is more 

 developed than the outer condyle. 



(b) The Tibia and Fibula. 



The fibula is produced proximally into a process like the ole- 

 cranon-process of an ulna : a character which may be present, but is 

 not clear in the nlna itself. The tibia is a stout bone, while the fibula 

 is comparatively slender. In both these Monotreme characters, as 

 well as in form and curvature, there is some approximation to the 

 tibia and fibula in Cryptobranchus, the giant salamander of Japan. 



Tl.o proximal end of the tibia, which is sub-reniform, is divided 

 into two surfaces, corresponding to the two condyles of the femur 

 by a deep vertical groove. The fibula does not lie in the groove of 

 the tibia, but articulates by a definite surface with the hinder 

 margin of the proximal end of the bone. The sides of the tibia 

 are concave, and upon this middle portion of the shaft a sharp 

 ridge crosses obliquely from its external or fibular side to its 

 internal border. The distal end expands transversely, is somewhat 

 compressed, but moderately convex, with the fibular margin gently 

 concave, and the internal border gently convex. The fibula slightly 

 overlaps the tibia in front, and articulates with it by a small surface. 



The fibul a contracts in the middle, and expands at the proximal 

 end somewhat abruptly. Its proximal articular surface is oblique 

 and concavely excavated, forming a concavity for the femoral con- 

 dyle, though the fibula does not appear to be prolonged proximally 

 appreciably beyond its articular surface. The internal border of 

 the bone is concave ; the external border is straight. The bone 

 maintains a nearly uniform thickness from end to end, becoming 

 somewhat flattened externally ; it is margined laterally by a slight 

 ridge in its lower half, which helps to make the distal end of the 

 bone very slightly concave. 



In Pareiasaurus the proximal end of the fibula appears to have 

 been greatly expanded, and terminated backward in a vertically 

 compressed plate. It is only known from the left fibula of Pareia- 

 saurus Baini, found resting horizontally on the spines of the vertebrae 

 in advance of the sacrum. 1 That form of proximal end is essentially 

 a modification of the Monotreme type in its expansion, but it shows a 

 certain resemblance to Aristodesmus, in which the bone is produced 

 a little forward as in Pareiasaurus and in Echidna. Both tibia and 

 fibula are intermediate between those of Pareiasaurus and those of 

 Echidna. 



See Phil. Trans. Eoy^Soc. vol. clxxxiii (1892) B, p. 314, fig. 2, reproduced 



specimen was removed from the rock. 



2u 2 



