86 EVOLUTION OF MAMMALIAN MOLAR TEETH 



styles to develop is the " metastyle," a reduplication of the metacone, the 

 well known " a-a " of Ptiitimeyer. 



In all ungulates in which the " mesostyle " is developed the external 

 cusps remain of the same size. In the tapirs no " mesostyle " appears, 

 yet these cusps are symmetrical ; but in the rhinoceroses, which also lack 

 the mesostyle, the first fact to note is the asymmetrical growth of these 

 cusps ; the metacone is elongated while the paracone is reduced and 

 crowded up against the parastyle. This point was observed by Cope in 

 seeking for a definition of the Ehinocerotidas in 18*75. The rhinocerotine 

 molar, whether of Hyrachyus, Amynodon or Acemthcrium, has the further 

 distinction that it is the only type in which a complete ectoloph is formed, 

 and second, as Cope has already observed, the asymmetry of the external 

 cusps is emphasized by the flattened metacone and conic paracone. 

 Figure 175 illustrates also the three projections from the ectoloph, 

 protoloph and metaloph, namely, the "crista," "antecrochet" and "crochet." 

 These, with the three " fossettes " formed by them, were noted and named 

 by Cuvier, and, as shown by Falconer, Flower, Lydekker and others, are of 

 great specific value. 1 We have already seen that Cuvier's term "fossette" 

 may be substituted for the " cement lakes " in the horse's molar. The 

 terms formerly adopted, or proposed, by Lydekker, 2 after English usage, 

 and those in German and French usage, have already been given in the 

 Table. 



There is another line of perissodactyls in which the metacone is 

 flattened but not elongated, and no complete ectoloph is formed. I refer 

 to the little Wasatch genus Heptodon (which Cope has erroneously placed 

 in the ancestry of Hyrachyus), also Hdaldes of the Bridger, an undoubted 

 successor of Heptodon, which Marsh was wrongly led to consider an 

 ancestor of the Tapirs. The molars, studied by our six differentia, are 

 found to differ from those of the rhinocerotine Hi/mclti/ns by the incom- 

 plete ectoloph, also by the shifting inwards of the metacone and con- 

 sequent shortening of the metaloph. In looking about for molars with 

 similar differentia, we find those of the true Lopliiodon of Europe, 

 L, isselensc, for example, stand nearest. 



Now, how shall we distinguish the early Tapirs ? First, there is no 

 mesostyle ; second, the paracone and metacone (as observed by Cope) are 

 both conic and symmetrical ; third, a feature of great importance, appar- 

 ently unnoticed hitherto, is that the protoloph and metaloph spring from 

 the anterior bases of the paracone and metacone, and not from near the 

 apices of these external cusps as in all molars of rhinocerotine affinity. 

 We find, as a general law, that where the external cusps are symmetrical 



1 As pointed out by Lydekker, the writer mistakenly transposed these terms " crochet " 

 and "antecrochet" in a former paper, Bull. Mus. Comp. Zool., 1890, p. 81. 



2 " Siwalik Rhinocerotida," Pal. Indica. 



