SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 55 



has the antero-internal ridge rather better defined. The third premolar 

 is nearly molariform, with the difference that near the inner end of the 

 anterior crescent is a well defined vertical ridge, which is more prominent 

 than in DiadiapJiortts and differs from that of LicapJirium in being a ridge 

 instead of a pillar. A remnant of the same structure appears in p^, but 

 the ridge is much smaller and is separated from the antero-internal border 

 only by a narrow groove. In perfectly unworn examples of p-g- a minute 

 internal cusp may be present at the hinder end of the posterior crescent, 

 a feature which is quite constant and better developed in Licaphriuni. 

 The fourth premolar is more completely molariform and, as already men- 

 tioned, the antero-internal ridge is reduced to a mere vestige. The cingu- 

 lum varies in development on the premolars, but is never very prominent. 



The molars are composed of two crescents, with the valleys opening 

 inwardly : on the inner side of the posterior end of the second crescent 

 is a small pillar, which is smallest in m^ and largest in m-g- ; in niy this 

 pillar is visible only in unworn teeth, as it is rapidly removed by abrasion : 

 m^ has in addition a small, but well defined talon. On the molars the 

 development of the cingulum varies much, even on the opposite sides of 

 the same jaw. Compared with those of Licaphriuni, the lower molars of 

 Proterotherium differ in being narrower and less massive, and in having 

 better defined valleys and much smaller postero-internal pillar. 



Milk Dentition. — I have seen no examples of the anterior upper tem- 

 porary teeth and only much worn specimens of the deciduous premolars. 

 Doubtless, however, the relationships are essentially the same as in Thoa- 

 therittm, which is described in a subsequent section. Both dp- and -, of 

 which the latter is much the larger, appear to be molariform in pattern. 

 All the mandibular milk-teeth are well displayed in the collection (PI. X, 

 figs. 15, 15a) and all of the antemolars have predecessors in the milk-series, 

 but there are some interesting differences of form between the temporary 

 teeth and their permanent successors. The median incisor (dig-) is larger, 

 more procumbent, more pointed and less chisel-shaped than i^, while 

 the lateral, dig, is very much smaller than ig and hardly larger than the 

 median, so that the milk-incisors of Proterotheriitni might easily be mistaken 

 for the permanent ones of Thoatheriitm. The temporary canine differs 

 from the permanent one merely in being somewhat larger. The first milk- 

 premolar is far larger than py and is implanted by two well separated 

 roots ; dpg- resembles p^ in form, but is longer and thicker and the ridges 



