SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 1 43 



has described a genus, Pseudocoslosoma, which he beheves to be distinct 

 from Theosodoji, but which is here regarded as a synonym of the latter. 

 In the original description ('qiw, 294) the very large, single inner root of 

 the upper molars is given as the diagnostic character, but, as already 

 pointed out (p. no), this is true of all the species of Theosodon. In a sub- 

 sequent paper ('04, 54) he writes that this animal shows numerous differ- 

 ences from the last-named genus, the only one of which mentioned is, 

 however, the presence of a pillar of the cingulum between the internal 

 cusps of the upper molars. As we have seen (p. 109), this is too variable 

 a character to be of generic significance. 



Theosodon lydekkeri Ameghino. 



(Plates XVII, Figs. 3, 3a ; XX, Figs. 6-7.) 



Theosodon Lydekkeri Am.tgh.mo ; Enum. sistematica, etc., 1887, p. 19. 



The original description of this species is founded upon fragments of a 

 large animal, the upper molar-series having an antero-posterior length of 

 73 mm. Ameghino's subsequent descriptions of much more complete 

 material, assigned to this species, all refer to considerably smaller indi- 

 viduals with upper molar-series 62 mm. in length, or less. Such a differ- 

 ence of size, nearly 17 per cent, would not of itself be a sufficient reason 

 for the specific distinction between the larger and the smaller individuals, 

 but two circumstances lead me to the conclusion that such a distinction 

 should be made, (i) The difference in size is very constant and in a large 

 series of specimens there is no perceptible tendency to approximation, 

 though, no doubt, a still larger series would show some such tendency. 

 This constancy is in very marked contrast to the conditions seen in Pro- 

 terotherium, in which size is extremely variable and wide differences are 

 connected by every intermediate gradation. (2) The milk-premolars of the 

 larger animals differ decidedly from those of the smaller, especially dp- 

 and -, which have much larger internal crescents and prominent posterior 

 pillars on the external face. Whether the permanent premolars show a 

 similar difference the known material does not enable me to say. In my 

 judgment, therefore, the name T. lydekkeri should be restricted to the large 

 individuals, which are, unfortunately, but very inadequately represented 

 in the collections. 



The following measurements are taken from a young, incomplete skull 

 (No. 15,717) in which m- has just come into use and m- is still concealed 

 in the jaw. 



