SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. I05 



MACRAUCHENID^. 



The second family of Santa Cruz Litopterna is that of the Macrauchen- 

 idas, which differ markedly in almost all points of structure from the Pro- 

 terotheriidas, yet with such unmistakable fundamental similarity as to leave 

 no question as to the propriety of referring both families to the same order. 



Aside from the little known and still problematical genus Adianthns, 

 the Santa Cruz representatives of the present family, so far discovered, 

 all belong to a single genus, Theosodon, which may be regarded, with 

 great probability, as directly ancestral to the Pampean genus Macrmi- 

 cJieiiia. In what follows, except with regard to the dentition, no account 

 is taken of Adianthus, of which only the teeth are known, and the fact 

 that it was a small animal. 



The species of Theosodon are much larger animals than the contempo- 

 rary Proterotheriidae and of very different appearance and proportions. 



In the present family there is no reduction in the number of teeth, the 

 formula being If, Ci, Pf, Mf, and there is no enlargement of incisors or 

 canines to form tusks. The first five teeth, from ii to pi, inclusive, are a 

 series with simple, recurved, compressed-conical crowns, which have a 

 curiously reptilian, almost crocodilian appearance. The second and third 

 premolars are smaller and simpler than the molars, while the fourth upper 

 premolar is nearly, and the fourth lower quite, molariform. The pattern 

 of the upper molars is, at first sight, very different from that of the Protero- 

 theriidae, but a closer examination reveals the identity of plan. As in the 

 latter family, the brachyodont crown consists of two external, concave 

 crescents, and two more or less conical internal cusps, the peculiarity of 

 pattern arising from the development of accessory crests and ridges and 

 the great elevation of the cingulum at the antero-internal angle. In 

 moderately worn teeth two or three enamel lakes are characteristic. The 

 lower molars are very similar to those of the Proterotheriidas and are made 

 up of two crescents, in the posterior one of which is a very conspicuous 

 pillar, connected with the external wall by a spur. In Adianthus this 

 element is absent. 



The antemolars all have predecessors in the temporary series and the 

 milk-teeth, are, in general, like their permanent successors and only the 

 last deciduous premolar, in each jaw, has the molar pattern. 



The skuH is long and narrow, with slender brain-case and rather short 



