MUSEUM OF COMPARATIVE ZOOLOGY. 



91 



^a.cr 



and metacone, to which is joined the anterior pillar (see Mesohippus, p. 88); it 

 may be called the ectoloph. As the anterior crest is formed by the union of the 

 protocone, protoconule, and paracone, it may be termed the protoloph. The 

 posterior crest, which unites the primitive metacone, the metaconule, and the 

 hypocone, may be termed the metaloph. The 

 outer surface of the ectoloph in the primitive 

 molar of the Rhinoceros is marked by three 

 vertical ridges corresponding to its three prim- 

 itive component elements, me, pa, ap ; one or 

 all of these disappear in the flattening of the 

 surface. It will be observed that nothing cor- 

 responding to the 'median pillar' of the su- 

 perior molar of the horse is developed. In the 

 lower molars (the paraconid disappearing), the 

 union of the metaconid and protoconid forms 

 the anterior crest, or metalophid, while the 

 hypoconid and entoconid unite to form the 

 hypolophid. 



The secondary enamel folds, which are de- 

 veloped from the three crests, bear a most in- 

 teresting analogy to those observed in the horse series, beginning with Proto- 

 hippus; they are outgrowths of the same regions of the crown and subserve 

 the same purpose. They are moreover of like value in phylogeny. The useful 

 descriptive terms introduced by Busk, Flow r er, and Lydekker, should be adopted 

 in part. 1 These secondary elements consist, first, of three folds projecting into 

 the median valley, one from the ectoloph, the crista ; one from the protoloph, 

 the crochet; one from the metaloph, the anticrochet. Secondly, the ecto- 

 loph unites with the posterior cingulum and metaloph. Thus the anterior and 

 posterior valleys may be cut off by the union of these folds into from one to 

 three ' fossettes,' precisely analogous to the ' lakes ' in the horse molar, except 

 that they are not filled with cement. 



The accompanying diagram is taken from a fossil molar figured by De Blain- 

 ville. (Osteogr. Gen. Rhin, Plate XIII.) It is remarkable in exhibiting all 

 the primary and secondary elements, for they are very rarely combined in a 

 single tooth. Similar accessory folds are frequently developed in the lower 

 molars. 



Figuhe 14. — Superior molar of 

 Rhinoceros (sp. indet.) X 5. After 

 Oe Blainville. 



1 The terms ' protoloph ' and ' metaloph ' are, however, substituted for ' anterior 

 eollis ' and 'posterior eollis ' of Lydekker. The term 'anterior pillar' = 'first 

 eosta,' and ' paracone ' = ' second costa.' The mode of evolution of the 'pillar' 

 must have been similar to that in the horses, where Lydekker has proposed this 

 term for the 'posterior pillar.' It is very appropriate, because the pillars in their 

 earliest development can be shown to rise independently from the cingulum (see 

 Mesohippus, p. 88), and not as folds of the main elements of the crown, as we should 

 infer from their fully developed stage. 



