FOSSIL HOESE REMAINS FROM IDAHO — GAZIN 299 



moderate wear the "heel" seen in Equus. With progressive wear the 

 angles of the protocone in all teeth become more rounded, the sides 

 more convex or sinuous, and the isthmus of attachment widens with 

 the shortening of the antero-external wall. In advanced wear (fig. 

 21, no. 12548) the protocone column generally becomes smaller and 

 more rounded, and the enamel reentrant defining the anterior portion 

 of the column is much reduced. At this stage the fossettes are simple 

 and U-shaped, and the teeth more strongly resemble those in 

 Pliohi'p'pus. 



The anteroposterior length and transverse width of the protocone 

 vary considerably between the teeth in the same dentition. The 

 width is noticeably greater in P^ and P^ than in the molars, usually 

 being relatively narrow in M^ The length is shortest in P- where 

 the column is simple with little or no portion projecting anterior to 

 the isthmus. The protocone increases in length posteriorly, although 

 P* in many dentitions shows a larger and more angular column than 

 in MS on account of the disparity in stage of wear between the two 

 teeth. In M^ the protocone reaches a length nearly half that of the 

 occlusal surface of the crown, and the postero-external wall is usually 

 more sinuous than in other teeth. The antero-external wall of this 

 column in M^ is commonly short and straight throughout wear and 

 projecting but very little anterior to the isthmus. 



The hypocone is simple and tapers to nearly a point postero- 

 internaliy in early wear but becomes blunt with moderate attrition. 

 It is separated from the metaconule by a slight constriction and does 

 not project lingually so far as does the protocone. In the premolars 

 there is generallj'- a spur projecting inward from the hypostyle forming 

 a section of the posterior wall of the tooth. In well-advanced wear 

 this spur may unite with the distal portion of the hypocone, thereby 

 isolating a small enamel lake (fig. 21, no. 12546). This spur from the 

 hypostyle is absent in the first two molars of nearly every dentition, 

 and only rarely in very advanced wear was the hypocone observed to 

 unite distally with the hypostyle, except through obliteration of the 

 enamel reentrant between them. In M^ however, the hypostyle 

 commonly unites with the distal end of the hypocone rather than with 

 the metaconule, except where the postfossette remains open. Union 

 is frequently made later, in more advanced wear, with the metaconule 

 also, leaving a lake as in some of the premolars. Occasionally in M^ 

 the hypostyle unites first with the metaconule (fig. 21, no. 12548), in 

 which case a closer resemblance is seen to other cheek teeth in the 

 series. 



Superior deciduous dentition. — The upper milk teeth are exhibited 

 in 35 or more of the skulls in the collections. Stages are represented 

 from that in which the teeth are erupting to the period where they are 

 replaced by the permanent series. A large proportion of these skulls, 



