40 EVOLUTION OF MAMMALIAN MOLAR TEETH 



intermediate between types I. and III. Example, Dromaiherium of 

 the American Triassic (Fig. 3). 



II. TricoiHu/nitf Type (Osborn, i>. cit., p. 242). The crown elongate, 

 trificl, with one central cone and two distinct lateral cones. The 

 fang double. Example, Tricoimdim (Fig. 8). 



III. Tritubercular Type (Cope). The crown triangular, surmounted 

 by three main cusps, the central cone placed internally in the upper 

 molars and externally in the lower molars. Example, the lower 

 molars of Spalacot/K'i-ium (Fig. 11) and Asthenodon (Fig. 35). This 

 type is rare in its primitive condition as above defined. 



The upper and lower molars are alike in types I. and II. : in 

 type III. they have a similar pattern, but with the arrangement of 

 the homologous cups reversed. These types are all primitive. In. 

 the following sub-types, the primitive triangle forms the main portion 

 of the crown, to which other " secondary " cusps are added, the homo- 

 logies of which in the upper and lower molars are somewhat doubtful. 

 Parallel and with an intimate relation to the addition of the secondary 

 cusps, is the division of the tritubercular into a secodont and buno- 

 dont series, according to the assumption of a purely cutting or 

 crushing function. In departing from the primitive type, the upper 

 and lower molars diverge in structure, and the homologies of the 

 secondary cusps in each are somewhat doubtful. 



Lower Molars. 



A. Tiilinrnhi)' Scdorial, Sub-type (Cope). () The primitive triangle 

 elevated and its cusps connected by cutting crests ; a low posterior 

 heel. (b) This type embraces a qmnquetubercular form in which 

 'the heel consists of two cusps, an internal and external. 1 (c.) In 

 the Bunodont series it develops into the quadritubercular form, by 

 the loss of one of the primitive cusps, 



Upper Molars. 



B. Trilnln'iri<l-r. (a). The primitive triangle in the secodont 

 series purely tricuspid. (b) This embraces a quinguetubercular form 

 in which " intermediate " tubercles are developed, both in the Seco- 

 dont and Bunodont series. (c) In the Bunodont series a postero- 

 internal cusp is added, forming the sexitubercidar molar. 



NOMENCLATURE OF THE CUSPS. As above stated, there is no doubt 

 about the homologies of the three "primary" cusps (protocone, para- 

 cone, metacone) in the upper and lower molars. They may be given 

 the same terms, with the arbitrary suffix id, to distinguish the 



Naturalist, April, 1883, p. 407. 



