SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. Ill 



vertical ridge, or incipient deuteroconid, divides the surface into two 

 shallow concavities. The cingulum, both internally and externally, is 

 more prominent than in p2. The fourth premolar is quite molariform, and 

 is composed of two crescents, but there is no pillar or spur in the internal 

 concavity of the posterior crescent, and the internal cingulum opposite the 

 openings of the crescents is better developed. 



The lower molars form a series nearly equalling in antero-posterior 

 length the premolar series, m2 is the largest and ms the smallest of the 

 three. All have the same crown-pattern, with two antero-posteriorly 

 directed crescents, opening inward ; in the inner opening of the posterior 

 crescent is a conspicuous spur, which, in perfectly unworn teeth, is seen to 

 be a vertical pillar, connected with the external wall by an obscurely ser- 

 rate ridge, above the level of which the unabraded apex of the pillar rises. 

 In such teeth, it may be plainly seen that the forward crescent is shorter 

 antero-posteriorly and more elevated dorso-ventrally than the hinder one, 

 though the difference in height is speedily obliterated by wear. 



Milk Dentition (Pis. XVII, figs., 3, 3^ ; XVIII, i, i«).— The tempo- 

 rary teeth differ but little in form from their permanent successors and are 

 32 in number ; the formula is : Dif, Dct, Dpf , all of them being changed. 



A. Upper Jaw. The deciduous incisors are much smaller than the 

 permanent ones, with crowns which are lower vertically, but relatively 

 more elongate antero-posteriorly than the latter and acutely pointed. The 

 canine is of the same shape as the incisors, but larger. The first milk- 

 premolar is an enlarged canine in form, but differs from its permanent 

 successor in having two roots, which demontstrates that all of the teeth 

 of the temporary series are shed and replaced. The second milk-pre- 

 molar differs somewhat in form in different species. In T. lallenianti it 

 is considerably smaller than p-, though of similar pattern, and has a single 

 internal crescent, formed by the elevated cingulum ; in T. lydekkeri it is 

 more like p- in form and has two internal crescents, of which the posterior 

 is very large, and a prominent posterior style appears at the postero- 

 external angle of the crown, an element which is rarely found in either p- 

 or - of the permanent series. The third milk-premolar is larger than dp- 

 especially in transverse width, and differs from its permanent successor 

 chiefly in the presence of a very conspicuous posterior and a smaller 

 anterior style on the external face. The fourth milk-premolar is more 

 completely molariform than p-, in having two well-defined internal cusps 



