52 EVOLUTION OF MAMMALIAN MOLAR TEETH 



2. That the protocone is invar/<tl>/// the nntd-ior lateral (antero-externai) 

 citxji n/ tin' lower rmtliti's n<l f//<- anterior Unt/nn] (antero-internal) cusp vn 

 the H /trier molnr*. The former part of this proposition is now almost self- 

 evident. It is absolutely proven in such a series as we see in Figure 38, 

 and is now corroborated by the embryological researches of Taeker and Rose. 



As to the present position or homologue of the reptilian protocone in 

 the upper mammalian molar there is relatively, I admit, more room for 

 doubt, mainly for the reason that fossil upper jaws are very scarce. If, 

 as held by Fleischmann and Rose, the antero-externai cusp is the protocone, 

 then the whole system of homologies held by Cope and myself falls to the 

 ground. Let us look at the evidence : 



First : In the numerous upper jaws of Triconodon (Figs. 7, 8) of the upper 

 Jurassic, the main cusp is always the middle one of the three, correspond- 

 ing with the large middle cusp of the lower molars which we know to be 

 the protocone. Second : In the upper molars of Spalacotherium * 

 (Jurassic), in which the lower molars are of the simplest tritubercular 

 type (Fig. 36, No. 4), the most prominent cusp by far is the antero-internal, 

 supporting my view. Third : In all the Amblotheriidae t of the upper 

 Jurassic there is a triangle of cusps in both upper and lower molars, in 

 each the apex is formed by the most prominent styliform cusp, this is 

 antero- external in f/ir lower molars ami antero-internal in the /<j>j>/',' molars. 

 Is it at all probable, at this early period, when the protoconid is still the 

 most conspicuous cusp in the lower molars that a corresponding cusp of 

 the same form, but reversed position, invariably found in the upper 

 molars is not homologous ? According to the Fleischmann-Rose view it 

 is not, but the main lower cusp is homologous with one of the spurs of 

 the main upper cusp. Fourth : There are other important grounds of a 

 mechanical nature, Starting with the study of modern, instead of the 

 oldest fossil forms, Fleischmann has, I believe, reached not only an 

 erroneous conception of the homologies of the separate cusps, but of the 

 equally important homologies in the functional regions of the upper and 

 lower crowns. In each we may distinguish two regions : 



The elevated primitive triangle (trigon) with a primitive cutting, 

 piercing or sectorial function. 



The depressed heel (talon), with a primitive crushing or grinding 

 function. 



In the earliest stages the upper and lower molars were simple 

 triangles of cusps, as in the modern Cape Mole, Chrysochloris. J The lower 

 molar had the apex (protoconid) turned outwards and the base (para- 



* [Regarded as a synonym of Percdestes. (See p. 35 and Fig. 12.) ED.] 



t [Represented by Kurtodon (Fig. 13, p. 26), Dryolestes (Fig. 14, p. 26). ED.] 



J [According to the views expressed on pages 124, 126, 227, the form of the molars 

 in Chrysochloris is entirely secondary. En.] 



