1 2 PATAGONIAN EXPEDITIONS : PALEONTOLOGY. 



1. Teeth more massive ; upper premolars less differentiated ; upper molars with inner cusps 

 partly connate ; pillar in posterior crescent of lower molars large ; animal stouter. 



Licaphriuvi. 



2. Teeth with thinner cusps ; upper premolars more complex ; upper molars with inner 

 cusps separate ; pillar of lower molars very minute and vestigial ; animal more slender. 



Proterothermfit. 



B. Feet monodactyl ; internal cusps of upper molars confluent ; lower lateral incisor (ij) but Httle 



larger than median (ig-). Thoathermm. 



The mutual relationships of these four genera are reasonably clear. 

 Thoatheriuin is in almost all respects much the most advanced and spe- 

 cialized and must have diverged from the others early in the history of the 

 family. One primitive feature, the subequal size of the lower incisors, 

 Thoatherhmi retains, and that this feature is really a primitive one, is 

 rendered very probable by the fact that in the other genera the milk-incisors 

 are of this type. Proterotherium and Licaphrium are very closely related 

 and may be taken as the central line of the family ; the former is somewhat 

 more advanced, as is shown in the more complex premolars and in the 

 reduction of the pillar to vestigial proportions. In dentition, LicapJiriitm 

 more nearly represents the primitive condition of the family than any of 

 the other genera. Diadiaphorus is another divergent line, characterized 

 by the complete loss of the pillar from the lower molars, and the notable 

 shortening of the nasal bones, for most of the species also by a greater 

 bodily stature. 



DIADIAPHORUS Ameghino. 



(Plates I-V; XIII, Fig. 9.) 



Diadiaphorus Amegh.; Enum. Sistematica, etc., 1887, p. 20. 

 Bunodontheriiim Mercerat ; Revista del Museo de La Plata, T. I, 1891, 



P- 450- 

 Oteomeryx Mercerat, in part; Ibid., p. 465 {^fide Ameghino). 



This genus includes the largest representatives of the family which are 

 found in the Santa Cruz beds, and is characterized (i) by the very com- 

 plete separation of the internal cusps of the upper molars, which have 

 nearly square crowns ; (2) by lower molars without talon, or postero- 

 internal pillar ; (3) by the much shortened nasal bones. 



Dentition (Pis. II, figs. 2-4 ; III, figs. 1-5, 8). 



A. Upper Jaw. The single upper incisor is large and caniniform, some- 

 what recurved and acutely pointed ; it is of triangular cross-section, with 



