194 Bulletin American Museum of Natural History. [Vol. XXVII, 



It regards a cusp primarily as an org-anic member of a whole crown, and 

 secondarily as an independent unit of structure. 



(/') Each cusp develops, it is true, its own individual shape, but this 

 does not disprove its correlation, in the strictest Cuvierian sense, with other 

 cusps or parts of the same organism. Cusps often serve in pairs or in series, 

 as fulcra, across which resistent material is broken {e. g., Palceosi/ops), and 

 in this instance a cusp may be correlated in form and position with a cusp 

 or with a depression on an adjacent molar as well as on an opposing molar. 

 In many dentitions, breaking, triturating and cutting are accomplished 

 through so complex a system of levers that sometimes hardly any two cusps 

 on a crown may have the same work to do, and the cusps assume different 

 shapes, as in upper molars of Meniscotherium. 



There may well be a principle of orthogenesis and of independent evolu- 

 tion operating "in every part of every organ" (Osborn), but it is obvious 

 that this tendency can be expressed only in so far as the very complex 

 mechanically adaptive conditions will permit; since in any organism every 

 part is more or less intimately environmental to every other part. 



Additional Notes on the Trituberculata. 



Peralesfes. Evidence is given below (p. 174) to show that Peralesfes 

 does not belong with Spalacotherium- and is a member not of the Tricono- 

 donta but probably of the Trituberculata (p. 176). However, the upper 

 molars designated under the term Peralesfes differ considerably from any of 

 the upper molars known in the Tritubercidata, namely those of Dicrocyno- 

 don, Kurfodon and Drijolesies. They also differ in several characters from 

 the hypothetical upper molars which have been inferred (p. ISO), from a study 

 of the lower teeth, to have pertained to Amphitherimn. Peralesfes may how- 

 ever be allied to Peramus, Lepfocladus and Peraspalax. These are known 

 from lower teeth, which agree in several characters with the hypothetical 

 type inferred (p. 174) for the lower teeth of Peralesfes. 



In brief it seems probable that the family Peralestidte Osborn (1888, 

 p. 247) is a member of the Trituberculata, and is a valid family, although 

 later merged by Osborn (1888.2., p. 301) in the Triconodontidae. 



Kurfodon. In this genus, the type of the Kurtodontidae of Osborn 

 (1888, p. 234), the upper molars (see Osborn, 1907, p. 26, Fig. 13) are 

 extremely wide and in the single known specimen Avell worn ; but they seem 

 to represent an exaggerated form of the Dryolesfes type with which they 

 agree in the possession of a parastyle, a centro-external paracone, a high 

 pointed protocone antl a slight internal cingulum. The worn molars as 

 figured show a transverse ridge running out from the protocone analogous 

 to the similarly placed ridge in Diademodon brachytiara and their excessive 



