230 



GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



It may be objected that most of these are generahzed dental characters of Eocene primates and do 

 not give a very detailed picture of the precise characters of the teeth in the ancestral Adapidse; but if 

 it be admitted that this reconstruction is supported by the available evidence, it would certainly be injudi- 

 cious to attempt to carry the process into the finer details which invariably impart specificity to actual 

 specimens. 



Our conception of the ancestral characters of the dentition in the primitive Adapidae should be clearly 

 adjusted to our conception of the ancestral skull characters and vice versa. And here it becomes neces- 

 sary to consider the probable relations of the ancestral Adapidse to the Tarsius-\ike primates of the Lower 

 Eocene. These, so far as known, are mostly very small animals, with large orbits, delicate muzzles, a 

 wide expanded brain-case with slight or no sagittal crest, malars delicate and widely separated from lacry- 

 mal, a wide rounded occiput, much enlarged bullae, wide and triangular upper molars, more or less crowded 

 or reduced incisor and premolar series, often with one or another of the lower incisors enlarged and pro- 

 cumbent and with the lower canines either reduced or enlarged procumbent. Although this group of 

 primates in the known palseontological record is nearly as old as the oldest of the Notharctinse, the writer 

 has little hesitation in regarding them as in all characters more specialized than the hypothetical ances- 

 tors of the Adapidse described above. Probably more or less nocturnal, and with many relatively advanced 

 specializations for the quick pursuit of hardshelled insects in the trees, they were among the earliest specia- 

 lized side groups of the primates, with precociously enlarged brains and sense organs and correspondingly 

 specialized skull characters and with aberrant speciaUzations of the dentition. 



The hypothetical ancestors of the Adapidse or even the most primitive Lower Eocene members of 

 the Notharctinse on the contrary had more slowly and conservatively developed the general primate 

 tendency to enlarge the brain-case; they had also begun to develop insectivorous-frugivorous specializa- 

 tions in the dentition. The little that is known of the skull of the oldest members of the Lower Eocene 

 Notharctinse indicates that in these small animals the brain-case and orbits were somewhat larger and 

 all the muscular crests less extremely developed than in their descendants of the Middle Eocene. It is, 

 in fact, a well recognized general rule, cited by Dr. Stehlin (1916, p. 1526), that small mammals have 

 relatively larger brains, less elevated sagittal and lambdoidal crests than those of their large-bodied rela- 

 tives, the species of Marmosa and Didelphys as well as the genera of the modern Lemuridse furnishing 

 beautiful examples of this principle. There are also Eocene genera of somewhat doubtful position {Pro- 

 nydicehus, Aphanolemur) , with the brain-case less expanded than in the known Tarsius-like primates, 

 but more expanded than in any of the typical Adapinse and Notharctinse. Of these Pronycticehus has 

 the malar region more like that of the Adapidse ; it has retained the primitive dental formula ^ and its 

 dentition as a whole is of an extremely generalized character. It was formerly classed by the writer as a 

 primitive member of the Adapidse, using that term in its widest sense, and was regarded by its describer 

 as ancestral to Nydicehiis. Whatever its more precise systematic position may be it tends to confirm 

 the view that the common ancestors of the Tarsius-\\k.e group and of the Adapis-Nothardus group were 

 small animals with a moderately expanded brain-case and with all the dental characters assigned above 

 to the ancestral primates. 



The excessive development of the sagittal and lambdoidal crests in the later Adapinse and Notharctinse 

 is not regarded by the writer as a part of their heritage from small-brained Mesozoic placentals. They 

 are more probably progressive specializations correlated with progressive expansion of the muscles of 



1 Although the incisors are lacking the very primitive number and character of the canines and premolars warrant the expectation 

 that the incisors were equally primitive. 



