258 D. M. S. Watson — Jyivibs of Lystrosaurus. 



from Harrismith, O.F.S. This shows the lower end of the tibia and 

 fibula, the tarsus, and some metatarsals and phalanges. The tibia is 

 not at all compressed, the lower articular face being almost circiilar. 

 The fibula is a much more slender bone than the tibia ; its narrow 

 shaft, however, flares out somewhat to the distal articular face, 

 whicli is flat. There are two tarsals, nearly spherical masses of very 

 spongy bone, with flattened dorsal and ventral siirfaces ; these two 

 bones enclose the usual foramen, and are so situated in relation to 

 the fibula as to show that that bone articulated witli both of them. 

 In this specimen and in another quite similar one, al?o in the British 

 Museum, the large articular face of tlie tibia does not face any bone, 

 and it seems quite certain that a larg6 part of it only articulated with 

 a cartilage during the life of the animal. The arrangement is, in 

 fact, extraordinarily like that of Limnoscelis as figured by Williston 

 {Amer. Journ. Sci., vol. xxxi, p. 396). The most reasonable 

 explanation seems to be that the two bones are the fibulare and 

 intermedium, the tibiale being unossified! This interpretation is 

 supported i)y the very curious fact that in Plesiosaurians, where the 



Fig. 3. The lower ends of the tibia and fibula, the complete tarsus, three 

 metacarpals, and some phalanges of the left side of Lystrosaurus. From 

 Harrismith, O.F.S. X J. 

 intermedium is always separated from the tibiale, that bone does not 

 ossify in F. Eatvkmsi, and is only represented by a very small 

 nodule of bone in P. rugostis. 



The great amount of cartilage remaining unossified about both 

 Limnoscelis and Lystrosaurus is rather remarkable, as the Seals and 

 Sirenia, which seem to offer the best comparison from the point of 

 view of probable habits, are well ossified. 



The three metacarpals preserved are slender, long-shafted bones, 

 little compressed, and tlieir proximal ends are very close to the two 

 ossified tarsals, so that the cartilaginous distal tarsals were probably 

 much reduced. The few phalanges preserved are similar to those of 

 the hand. 



One of the incomplete feet of Lystrosaurus in Grahamstown shows 

 four phalanges on two neighbouring digits, so that in the foot this type 

 exceeded the normal mammalian number of 2, 3, 3, 3, 3 of phalanges 

 which occur, as shown by the specimen just described, in the hand. 



