Eevieus—Prof. E. F. Oshorn — Molars of Perissocladyla. 319 



of the Horses. To this statement we must take the present oppor- 

 tunity of objecting. The term posterior 'pillar (although it has been 



hy. ml. pr. pi. 



Fig. 3. — Three right upper cheek-teeth of Sipparion. 

 letters as in Fisr. 2. 



ml. metaconule ; 



subsequently employed by the writer) was first proposed by Prof. 

 Hnxley ("Anatomy of Vertebrates," p. 354, 1871), and was applied 

 to the tubercle in the molars of the Horses marked liy. in Figs. 3 and 4 

 (these figures are unfortunately from the opposite side of the jaw to 

 those figured by Prof. Osborn). This tubercle corresponds with the 

 one similarly lettered in Fig. 2 (the hypocone) ; 

 and has, therefore, nothing whatever to do 

 with the one marked pp. Under these circum- 

 stances we propose to substitute the term 

 posterior prominence for the tubercle pp. Again, 

 the tei'm anterior pillar was proposed by Prof. 

 Huxley, and adopted by the writer for the 

 tubercle marked pr. (the protocoue) in Figs. 

 2, 3 ; and it is, therefore, quite inadmissible 

 to apply it to a. in Fig. 2. Under these 

 circumstances we would suggest that the 

 vertical ridges marked a. m. and p. in Fig. 2 

 (which become still more prominent in the 

 teeth of the Horses), should respectively be 

 designated the anterior ridge, middle ridge, and posterior ridge. 



The figures sufficiently indicate how the equine molar has been, 

 evolved from that of Ancliitherium by the development of one pro- 

 jection from the protoconule to join the metaconule, and of another 

 from the latter connecting it with the posterior prominence, and 

 thus with the metacone ; the lengthening of the crown and the 

 filling of the valleys with cement being a concomitant process. 

 The result of this evolution has been to form complete crescents 

 in the equine molar, of which the hinder one is always connected 

 with the hypocone {liy.) ; while the anterior one may either be 

 connected with the protocone {pr.), as in Equus, or completely 

 isolated, as in Hipparion. This has also entailed the division of 

 the median and posterior valleys of the Ancliitherium molar into an 

 inner and an outer moiety. 



The remaining type of Perissodactyle molar we have to notice 

 is that of the Ehinoceroses (Fig. 5). Here Prof. Osborn has intro- 



FiG. 4. — Right upper 

 molar of Equus sienonis ; 

 letters as in Fiff. .3. 



