202 



GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



The evidence offered by the milk teeth of Adapts {op. cit. p. 1532) for the view that the primitive 

 canine was brachyodont and much more premolariform than in Pelycodus frugivorus has in the writer's 

 judgment but httle bearing on the question, because in spite of traditional reliance on the Biogenetic 

 Law, it remains to be proved in each case whether the milk dentition is more primitive than the perma- 

 nent dentition or whether it is specialized for its own purposes as larval structures frequently are. More- 

 over the total inability of the young to defend themselves is in harmony with the inoffensive character 

 of the deciduous canine. 



Dr. Stehlin (op. cit., pp. 1531, 1532) cites the mode of evolution of the front teeth in the Eocene peris- 

 sodactyls and artiodactyls for the purpose of showing that abnormal types of " Vordergebiss " are always 

 derived from forms with low, small and subpremolariform canines, as in the Oreodontidse, etc. To this 

 it may be replied: first, that as shown by their astragalus the oldest bunodont Artiodactyla of the Lower 

 Eocene ^ were already fairly well specialized in the Artiodactyl direction and that until their Paleocene 

 and Upper Cretaceous ancestors be known it will not be safe to affirm that the small canines of Eocene 

 Artiodactyls are a primitive and not a retrogressive character; indeed, the compressed form of the pre- 

 molars, the elongate slender ramus, and the cropping character of the incisors all appear to the writer 

 rather to be herbivorous specializations from a primitive type with stouter, shorter rami, less compressed 

 premolars and less reduced canines. Secondly, it does not seem necessary to go outside of the Adapidae 

 themselves for information concerning the early form of the canines in that family. As already noted 

 in the Lower Eocene species Pelycodus ralstoni and P. frugivorus of the Notharctinse and in the Lutetian 

 species of the Adapinse, namely Adapts sciureus and A. rutimeyeri, the lower canines, although sharing 

 some characters with the premolars, are on the whole more caniniform than premolariform and were 

 much larger than the first premolar. 



The peculiar specialization of the tongue and lower front teeth in the Lemurida?, which is remotely 

 paralleled among the ruminants and oreodonts, illustrates several important facts and principles, which 

 have a direct bearing on the problem of the degree of relationship between different groups of Eocene 

 lemuroitls and the modern Lemurida\ The principle of convergent evolution between adjacent 

 TEETH is well illustrated by the fact that the lower incisors and canines are doubtless now much more alike 

 than they were in the remote ancestors. The well-known principle of Change of Function involving 

 a no less radical change of form is also illustrated by the incisiform habitus and function assumed by the 

 canines. A corollary of this is the principle of substitution and usurpation; the canine taking the 

 place of a long lost incisor, the second lower premolar performing the functions of a lower canine and 

 usurping the position of the first lower premolar. A corollary of the preceding principle is that the 

 direction of evolution is subject to radical changes, involving the loss of old and the develop- 

 ment of new and quite different tendencies. Whatever may have been the precise character of the lower 

 canines, as long as they retained their normal articulating relations with the upper canines they must 

 have been considerably different in form and function from the incisors which they now resemble. 



This discussion may be concluded by an impartial summary of the opposing conclusions of Dr. Stehlin 

 and the writer concerning the form of the front teeth in the ancestral Primates and especially in the 

 ancestors of the Lemuridse. 



Dr. Stehhn (1916, p. 1530) rejects the suggestion of the present writer (1915, pp. 424, 425) that the 

 lemurid specialization of the front teeth has been derived from the conditions illustrated in Notharctus; 

 he believes that the canines in this genus were too specialized and after long search he can find no well- 



' Cf. Sinclair, W. .1., 1914, Bull. Amer. Mus. Nat. Hist., XXXIII, \>. 271, fig.s. 3, 4. 



