TKITI:I:KI:< TLY ix rxci'LATKs 



Tin' Rhinoceros Molar. 



The peculiarities of the molars of A-phclop* [cf. Fig. 17-"., p. 181] will 

 be made more clear by a few observations upon the molars of the rhin- 

 oceroses in general. The three main crests of the lophodont crown 

 may now be distinguished in part by terms which express their 

 homologies with the elements of the sexitubercular superior and 

 quadritubercular inferior molars of the primitive ungulate, PTienacoduus. 

 In the upper molars, the outer crest is formed by the union of the 

 primitive paracone and metacone, to which is joined the anterior pillar 

 (see Mesohippus, p. 175); it may be called the cctoloph. As the anterior 

 crest is formed by the union of the protocone, protoconule, and paracone, 

 it may be termed the protolopli. 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 

 primitive component elements, rn>', pa, ap [pas] ; one or all of these disappear 

 in the flattening of the surface. It will be observed that nothing corre- 

 sponding to the ' median pillar ' of the superior molar of the horse is 

 developed. In the lower molars (the paraconid disappearing), the union 

 of the metaconid and protocouid forms the anterior crest or metalophid, 

 while the hypoconid and entoconid unite to form the hypol<>i>lii<l. 



The secondary enamel folds, which are developed from the three crests, 

 bear a most interesting analogy to those observed in the horse series, 

 beginning with Protohipjw.x [Euliippv*, Fig. 166]; they are outgrowths of the 

 same regions of the crown and subserve the same purpose. They are, more- 

 over, of like value in phylogeny. The useful descriptive terms introduced 

 by Busk, Flower, 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 proto- 

 loph, the crochet ; * one from the metaloph, the antecrochetJ Secondly, 

 the ectoloph 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. 



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

 and 'posterior collis' of Lydekker. The term 'anterior pillar '=:' tirst costa,' 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. 175), and not as folds of the main 

 elements of the crown, as we should infer from their fully developed stage. 



:: [Should have read antecrochet.T&D.] 

 I [Should have read crochet. ED.] 



