2 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



It is still a matter of some uncertainty as to the number of families rep- 

 resented among the Santa Cruz Litopterna. At one time or another, 

 Ameghino has referred to this order four families, the Notohippidse, Pro- 

 terotheriidae, Macrauchenidae (including the Mesorhinidas) and the Adian- 

 thidas. Of these, the Notohippidae, which Ameghino has latterly referred 

 to the horses, his order " Hippoidea" ('04, 33), I agree with Roth ('03, 

 33) in regarding as nearly allied, if not actually referable, to the Toxo- 

 dontia. As to the Proterotheriidae and Macrauchenidae there is no dif- 

 ference of opinion, they are clearly defined and unmistakable family 

 groups. The Adianthidas are so very imperfectly known, that their status 

 is still quite uncertain ; they are here merged with the Macrauchenidae, 

 from which a separation of them is, at present, at least premature. 



The Litopterna are an unusually interesting group of mammals, because 

 of the fundamental problems of descent which they raise, and a definite 

 determination of their systematic position and relationships will illuminate 

 many obscure questions of evolutionary philosophy. Especially is this 

 true as regards the great problem of parallel and convergent development, 

 the independent acquisition of similar structures in unrelated or remotely 

 related groups. 



The dentition, which is always brachyodont in all of the known members 

 of the order, differs considerably in the two Santa Cruz families, more par- 

 ticularly in the anterior region. In the Proterotheriidae the constant dental 

 formula is \\, Ct, Pt, Mf, and the single upper incisor and external lower 

 incisor (i- and \z) are enlarged, growing from persistent pulps, to four 

 small, sharp-pointed tusks ; the upper canine is lost and the lower is so 

 small as to be nearly or quite functionless. In the Macrauchenidae the 

 number of teeth is unreduced from that typical of nearly all groups of early 

 placental mammals, giving the formula I|, Ct, Pt, Mf. The incisors, 

 canines and foremost premolars are of subequal size and simple, com- 

 pressed conical form. 



In all of the Litopterna, even those of the Pleistocence, the premolars 

 are nearly all simpler in pattern than the molars. In the Santa Cruz 

 genera the fourth (and sometimes the third) lower premolar is quite molari- 

 form, and the fourth upper approximately so. The molars have a pattern 

 which has often been compared to that of PalcBotherittm, but, so far as the 

 upper teeth are concerned, the resemblance is not at all close and, indeed, 

 the plan is quite different from that of any of the lophodont Perissodac- 



