CHAP. VI] THE DENTAL SYSTEM OF THE PRIMATES 283 



the cases, and hence it is necessary to enter into brief notices of 

 each. 



The tritubercular hypothesis is inseparably connected with the 

 names of two American palaeontologists, viz. Cope and Osborn 1 , 

 and it is interesting to note that the theory is largely a 

 generalization from the experience of extended investigations of 

 the characters of fossil teeth. It was considered applicable to the 

 molar teeth of both jaws (upper and lower) and to the lower series 

 of premolar teeth ; but not to the upper premolar teeth. 



The theory has had an eventful history since Cope first ex- 

 pounded it some thirty years ago (1883). It has been extended 

 and modified. Certain details have been abandoned as a result 

 of the progress of anatomical and palaeontological science. The 



Prd Hd 



B 



Fig. 203. Molar teeth of Periptychus. A, upper; B, lower tooth. (From Osborn.) 

 The three "primitive" cusps in the upper tooth are labelled Pr (protocone), Pa 

 (paracone), and M (metacone) respectively. In the lower tooth (B) the three 

 "primitive" cusps are distinguished as Prd (protoconid), Pad (paraconid) and Md 

 (metaconid), the names in parentheses having been devised by Cope. 



essential parts of the theory have also been misapprehended, but 

 in the brief sketch which follows, I have made every endeavour 

 to present a fair account derived from the original sources so far 

 as possible. 



(i) As a conception, the Tritubercular theory seems to rest 

 upon Cope's recognition of a remarkable feature in the teeth of 

 certain very ancient fossil mammals found in New Mexico (Puerco 

 Eocene). The animals themselves are assigned to the earliest sub- 

 division of the Tertiary period (Basal Eocene). The peculiarity 

 thus observed consisted in the fact that the molar teeth commonly 

 presented three cusps arranged at the angles of a triangle (Fig. 203). 



1 Cf. Cope, Primary Factors in Evolution; Osborn, American Naturalist, 1888, 

 p. 1074; ibid. 1893 and 1897 ; Evolution of Mammalian Molar IV, th, 1907; Gregory, 

 Bulletins of the American Museum of Natural History, 1910. 



