214 EVOLUTION OF MAMMALIAN MOLAR TEETH 



(2) A Secondary Cone = metacone of ms, tritocone of pms = hypoconid 

 of lower ms and pms. 



(3) Three cinguluin cusps 



(a) anterior = deuterocone, minute in all except lower sectorial 



where it = the paraconid. 



(b) posterior = shear (metastyle) of sectorial. 



(c) centro-internal = protocone of <lpm 4 = heel (internal cingulum 



of m 1 ) = metaconid of lower teeth. 



This theory agrees with the " premolar analogy" theory and with 

 the theory of Gidley in identifying the paracone of the molars, the 

 protocone of the premolars, and the protoconid of the lower premolars 

 and molars as the primary cusp. 



Summary of Objections to the Embryological Theory. 



In reply to this strong presentation of the facts of embryogeny 

 the following points may be made: 1. It is possible that the superior 

 tritubercular molar arises independently of previous protodont and 

 triconodont stages ; but the evidence afforded by the lower molar 

 teeth of Spalacotherium, which are incipiently tritubercular, tends to 

 connect these stages (Figs. 11, 12, and p. 32 />). 2. We do not posi- 

 tively know the structure of the upper molars in Spalacotherium ; but the 

 presumption that they were somewhat similar to the lower molars is 

 strongly supported by the fact that in the closely related genus Tri- 

 conodon the upper molars are exactly similar.* 3. The supposed upper 

 molars of Spalacotherium, named Peralcstcs by Owen, although of some- 

 what irregular form, present a pattern reversing that in the lower 

 molars of Spalacotherium, namely, with a large internal cusp, with a 

 closely connected antero-external crest surmounted by a cuspule, and 

 with a freer and more detached postero-external cone. Figure 12 in 

 Chapter I., not heretofore published, was directly taken from camera- 

 lucida outlines of the upper molars of Peralcstcs shaded with pencil. 

 It shows that the earlier figures of Osborn and Woodward were only 

 partially correct, and it tends to demonstrate, apparently beyond a doubt, 

 the existence of a large, prominent internal protocone, and of an antero- 

 external paracone shelf as originally homologized by Osborn, a postero- 

 external conical metacone, and an external cingulum embracing the 

 outer side of the crown at the base corresponding with the internal 

 cingulum of the lower molars of Spalacotherium.^ 4. The comparison 

 of a series of zalambdodont (tritubercular) and dilambdodont (quadri- 



* [The presumed relationship between Triconodon and Spalacotherium is denied by 

 some authors. ED.] 



t [The supposed "protocone" of Peralestes might possibly be homologous with the 

 pai'acone of later mammals. ED.] 



