58 



ORIGIN OF TOOTH PATTERNS 



" talon " ; and this ledge became produced into two additional 

 cusps, the hypoconule or hypoconulid, and the ectocone or ecto- 

 conid. Thus the typical sextuberculate tooth of the primitive 

 Ungulate, and indeed of many primitive Eutherians, is arrived at. 



.. .frotocmid 



frolixoMiJ.s^ 



I \nutafnid 



fianuontd i x// , w 



Ayfotantd 



FIG. 39. Epitome of the evolution of a cusped tooth. 1, Reptile; 2, Dromathermm ; 

 3, Micrnconodon ; 4, Spalacotherium : me, metaconid ; pa, paraconid ; pr, proto- 

 conid ; 5, Amphitherium. (After Osborn. ) 



From this the still further complicated teeth of modern Ungulates 

 can be derived by further additions or fusions, etc. 1 On the other 

 hand, the development of the Primate molar stops short at the 

 stage of four cusps. 



That such a series can be traced is an undoubted fact. Every 

 stage exists, or has existed. But whether the stages can be con- 

 nected or not is quite another question. It is by three main 

 lines of argument that the view here sketched out in brief is 

 supported. In the first place, the tracing of the pedigrees of 

 many groups of mammals has met with very considerable success ; 

 and it is clear that as we pass from the living Horse and 

 Rhinoceros, with their complicated molars, to their forerunners, 

 we find that both can be referred to a primitive Ungulate molar 

 with but six cusps. Going still further back to the lowest 

 Eocene and ancestral type as it appears, Euprotoyonia, we still 

 find in the molar tooth the sextubercular plan of structure. We 

 can hardly get further back in the evolution of the Perissodactyles 

 with any probability of security. On the other hand, many facts 

 point to a fundamental relationship between the primitive Ungu- 

 lates and the early Creodonts. The latter frequently show plainly 

 tritubercular molars. Such Ungulates as Ewprotogonia and Proto- 

 f/onodon, though sex- or quinque-tubercular as to their molars, 

 have a distinctly prevailing trituberculism, when the size and 

 importance of three of the cusps is taken into account. But this 



1 e.g. the " protoloph, " "metaloph," etc. (see Fig. 36, p. 51), of the modern 

 Ungulate form of tooth. 



