300 PROCEEDINGS OF THE NATIONAL MUSEUM vol.83 



however, are at a stage in which the first true molar is just erupting. 

 In the course of replacement Dp^ is lost at about the time M^ is in early 

 wear and M^ is just erupting. The central incisors are replaced ap- 

 parently a little earlier, at about the time M^ is commencing to wear. 

 Dp^ and Dp^ are replaced in order, with P* beginning to wear at about 

 the same time or slightly ahead of M^ The lateral milk incisors are 

 somewhat slower to be replaced, with P erupting at about the time 

 P* and M^ begin to wear, at an age probably between 4 and 5 years, 

 and I^ is the last to appear. The permanent canine comes in at about 

 the time of I^ or 1^ in the males, perhaps earlier in the females where 

 this tooth is much smaller. 



The deciduous incisors are a little larger than in Equus grevyi, par- 

 ticularly in their labio-lingual width. The crowns are brachydont, 

 with shallow, occasionally cement-coated cups. The first incisors are 

 generally the heaviest and the third the shortest crowned and weakest. 

 The third incisor is also usually somewhat constricted transversely 

 near the posterior end. 



The milk canine was not observed in any of the specimens, although 

 most of the immature skulls show either an alveolus or a roughened 

 area at this point. In a few instances the size of the alveolus suggests 

 a tooth larger than in the modern horses observed. 



The three deciduous premolars (fig. 21, no. 12494 and no. 12520) 

 are of comparatively large size and with the anteroposterior length 

 relatively great. The teeth are mxoderately hypsodont but much less 

 so than the permanent cheek teeth. Accompanying the greater rela- 

 tive length of the occlusal surface the fossettes are elongate and the 

 protoloph is noticeably wide anterior to the prefossette. The teeth 

 are generally well cemented except in the fossettes where cementa- 

 tion is usually not complete, and in one instance (no. 13842) the 

 dentine was not completely developed in the cusp area. The meso- 

 styles on the outer walls of Dp^ and Dp* are compressed as in the 

 permanent teeth, but the area between the styles is more flattened 

 and the parastyles are relatively heavy though apparently not heavier 

 than in E. grevyi. The fossettes are generally simple and show a 

 plicate border highly variable as in the permanent teeth, with the pli 

 protoconule usually the best developed. The fossettes also show a 

 tendency to remain open. The anterior fossette in many dentitions 

 remains open with appreciable wear tlu'ough the postprotoconal val- 

 ley, especially in Dp-, with the pli protoconule projecting from the 

 protoloph and a highly variable pli caballin extending into the post- 

 protoconal valley from the crochet. The posterior fossette in Dp* is 

 commonly open posteriorly external to the hypostyle (fig. 21, no. 

 12494), and in a few instances, as in no. 12484, the postfossette in 

 Dp^ is open through the posthypoconal valley. 



