26 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 6l 



dentine of the tooth, Later the posterior valley lying between the 

 ectoloph and metaloph is cut off by the union of its posterior walls to 

 form the postfossette. Finally, by the union of the projecting ends of 

 the metaloph and the protoloph, another island is cut off. This is the 

 prefossette, the most anterior in position but the last to be formed. 

 By this union the last fossette has been formed and the tooth now has 

 a square crown with three central islands of enamel. No antecrotchet 

 is formed, at least no ridge is given off from the median border of the 

 protoloph, the union with the metaloph taking place at the extreme 

 tip of the protoloph. This condition takes place fairly early and 

 remains throughout most of the life of the tooth, the fossettes not 

 disappearing until the crown is worn nearly down to the roots. The 

 permanent premolars and the molars develop their folds precisely as 

 the milk molars, but have in addition to the enamel and dentine a 

 el 



m I 



Fig. 2. 



Fig. 3. 



Fig. 1. — Second upper molar of Ceratotkerium simum cottoni. 



Fig. 2. — Second upper premolar of Ceratotkerium simum cottoni, illustrating 

 position of lophs and fossettes and composition of cheek teeth ; both y 2 natural 

 size. 



Cr, crotchet; el, ectoloph; ;;;/, medifosette ; ml, metaloph; pf, prefosette ; 

 pi, protoloph ; ps, postfossette ; cement layer = diagonal ruling ; dentine = ver- 

 tical ruling; enamel = white. 



Fig. 3. — First lbwer molar of Ceratotkerium simum cottoni, illustrating posi- 

 tion of lophs and composition of mandibular cheek teeth; y 2 natural size. 



Hd, hypolophid ; pd, protolophid. Cement layer = diagonal ruling; dentine 

 = vertical ruling; enamel = white. 



heavy outer coating of cement. This layer is usually so thick that 

 when carried into the center of the crown by the formation of the 

 fossettes it completely fills these cavities. This abundance of cement 

 in the teeth is similar to the condition found in the Equidcu, and is one 

 of the marked di.< mguishing characters of this genus of Rhinoce- 

 rotidec. 



As the molars are erupted the premolars are pushed forward, the 

 eruption of each molar taking place opposite the termination of the 



