1894. ^^^ ^^^ PLATA MUSEUM. 121 



teeth are, moreover, exceedingly like those of the Perissodactyla, and 

 more especially the Rhinocerotidae, to which the members of this group 

 approximated in point of size. Although there seems to be but one 

 species of the first-mentioned genus and only two or three of the latter, 

 the list of synonyms in the case of Astrapotherium is of the usual 

 appalling length. As its name implies, the genus HomalodontotJieviwn 

 is characterised by the teeth being forty-four in number and forming 

 an uninterrupted series, with the canines not longer than the incisors. 

 Until recently this genus was known only by the teeth and jaws, but 

 the La Plata Museum contains numerous specimens of the vertebrae 

 and limb-bones. Among these, the humerus is remarkable for the 

 great development of its deltoid crest, which recalls that of the 

 wombat. 



A very different animal is the gigantic Astrapotherium, the type 

 species of which was originally described by Owen as Nesodon 

 magnum. In this creature the dentition is reduced, and each jaw 

 furnished with a huge pair of tusks, while the upper molars are 

 extraordinarily like those of the rhinoceroses. There are no teeth 

 between the huge upper tusks, which I have reason to believe 

 are incisors; but in the lower jaw there are three pairs of small 

 incisors, with curious spatulate crowns, situated between the pig-like 

 tusks, which are here clearly canines. Astrapotherium has been 

 placed among the so-called Dinocerata, but it is certain that such 

 resemblances as it presents to that group must be attributed to 

 parallelism, while its relationship to Homalodontotherium (as proved 

 by the limb-bones in the Museum) is perfectly clear. It cannot, 

 moreover, have any direct relationship with the Rhinocerotid^, so 

 that the resemblance of its molar teeth to those of that group is 

 again apparently due to parallelism. 



A third subordinal group of extinct Ungulates peculiar to South 

 America is represented by Macrauchenia in the Pampean deposits, and 

 hy Protevothevium and certain allied forms in the Patagonian Tertiaries. 

 These animals have been placed by some writers with the Perissodactyla, 

 but it is certain that Professor Cope is perfectly correct in regarding 

 them as representing a distinct suborder — the Litopterna. Agreeing 

 with the Perissodactyles in having an odd number of toes, with the 

 middle one symmetrical in itself, and likewise in the pulley-like upper 

 surface of the astragalus, these Ungulates differ from that group in 

 having both the carpus and the tarsus of the linear type,- and likewise by 

 the fibula articulating to a small facet on the calcaneum (as in the Artio- 

 dactyla). Moreover, in those cases where they are known, the vertebrae 

 of the neck are much elongated, and have the sides of the neural 

 arch pierced by the canal for the vertebral artery in a manner now 



2 It may be well to mention that in the linear type of carpus and tarsus the 

 bones of the two horizontal rows are set directly one over the other (as in the 

 Proboscidea), whereas in the alternating type the bones of the upper row are placed 

 over the divisions between those of the lower. 



