208 



GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



This gives evidence that the arrangement of the mental foramina in the Lemuridse is closely related 

 to or derived from the primitive arrangement shown in the Adapidse and that Lepilemur, as might be 

 expected, is the most primitive of the modern types. 



Skull 

 Plates LII — LV 



The comparative morphology of the skull of the Lemuridse, Notharctinse and Adapinse may now be 

 discussed. 



As compared with the Notharctinse, the skulls of typical Lemuridse exhibit the following important 

 differences. 



(1) They are much more advanced in evolution in respect of the following characters: 



(a) the brain-case is greatly expanded transversely and in consequence the sagittal and lamb- 

 doidal crests are typically lost, the interparietal is widely exposed on the top of the skull 

 and the interorbital constriction is nearly obliterated; the frontals extend further back- 

 ward over the parietals; 



(5) the orbits are relatively larger; 



(c) the lacrymal is extended on the face, carrying the lacrymal foramen in front of the orbit; 

 {d) the bony muzzle is often more or less truncate and in correlation with this the premaxillse 



are often widened and shortened anteroposteriorly ; 

 (e) the notch between the palatal border of the palatine and the alveolar region of the maxillary 



is converted into a tunnel; 

 (/) the zygomatic process of the malar sometimes extends back nearly to the articular eminence, 



while in the Notharctinse it apparently ended weU in front of that point; 

 ig) the pterygoid fossa? are typically more expanded than in Nothardus; 



(h) the buUse are frequently more expanded and the mastoid is sometimes inflated (Lepilemur, 



Microcebus) ; 



(i) the fossa subarcuata on the encephalic surface of the petrosal is more expanded. 



With the possible exception of /, every one of these, in the judgment of the writer, is a progressive 

 character and not one excludes the notharctine type of skull from remote structural ancestry to the lemurid 

 type. 



(2) Retrogressive features in comparison with the Notharctinse include the weakening of the zygo- 

 matic arches, the reduction of the temporal flange of the squamosal, the weakening of the premaxillse 

 and of the suborbital tubercle for the masseter, the loss or reduction of the prominent mastoid tuberosity. 

 Here again these characters in the opinion of the writer are all relatively late acquisitions and by no 

 means exclude the Notharctinse from remote structural ancestry to the Lemuridse. 



Characters a, h, c, d, and e of the progressive series and all the retrogressive characters also distinguish 

 the typical Lemuridse from the Adapinse, but do not exclude them from structural derivation from the 

 latter; characters g, h, and i are already developed in the Adapinse whence they may have been transmitted 

 to the Lemuridse. 



In the course of his description of the skulls of Adapts parisiensis Dr. Stehlin records numerous 

 resemblances with and differences from Lemur besides those already noted. In addition to the great 



