220 



GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



extension of the lacrymal and the preorbital position of the lacrymal foramen in these animals are regarded 

 by the writer as secondary, not primitive characters. Nor can the view that "Adapts and Nothardus 

 exhibit advance in the reduction of the lacrymals" be accepted. The whole face of Nothardus appears 

 to be in very primitive condition with characters ancestral to those of both the lemuroid and the 

 platyrrhine groups. 



Setting aside for the present the debatable question whether the Old World and the New World 

 series have been derived from the same stock, and noting that Omomys and Washakius probably belong 

 with the Tarsius-\ike Primates and have nothing to do with the Cebida?, we find, after an extended com- 

 parison between Nothardus and the Platyrrhini, that the fuller evidence supports Dr. Wortman's con- 

 clusion that the "interval. . .is not greater than one would be reasonably led to anticipate between an 

 ancestor of Upper Eocene time and a living descendant." There can be no doubt of this statement, 

 with reference to the great majority of the characters separating the modern Platyrrhini from Nothardus, 

 ^\'hich are nearly all correlated directly with the progressive enlargement of the brain, with the shortening 

 of the face and with the mingling of retrogressive and progressive changes. Nothardus also shows certain 

 important special resemblances with the Platyrrhini in the form of the incisors, in the formulae of the adult 

 and deciduous dentitions and in the direction of evolution of the premolars ; this last resemblance is espe- 

 cially striking if we compare the lower premolars of Nothardus pugnax with those of Alouatta. If the 

 posterointernal cusps of the upper molars of the Platyrrhini be pseudohypocones, as the conditions in 

 Callithrix apparently indicate, another and very important resemblance must be recorded. 



There are other differences between Nothardus and the Platyrrhini, which certain investigators may 

 be inclined to interpret as divergent specializations, tending to exclude the former from structural ancestry 

 to the latter. Of first importance is the difference in the relations of the tympanic ring to the bulla. 

 A working hypothesis as to the probable history of the tympanic region in the ancestors of the Platyr- 

 rhini is as follows. 



The primitive mammalian condition (in which the tympanic ring was entirely outside of the bulla 

 and which is still illustrated in the foetal Chiromys ^) had been replaced by the stage illustrated in Adapts 

 and N'othardus, in which the expanding hypotympanic sinus had overgrown ventrally the tympanic 

 ring. The rapid transverse expansion of the brain initiated a secondary uncovering of the ring in the 

 following manner. The tympanic ring, retaining its connections with the entoglenoid and post-tympanic 

 processes, shared in the general lateral displacement of this region, while the bulla itself remained fastened 

 to the side of the basioccipital and to the posteroexternal corner of the basisphenoid. The ring wa& 

 first exposed at the aperture of the bulla, then protruded from it and then began to overlap it. Mean- 

 while the cavity of the bulla itself diminishes, it retains the cancellous condition which usually precedes 

 the resorption of its tissue, while the tympanic enlarges, spreads over the bulla and becomes cancellous. 

 In favor of the view that the overlapping of the ring by the bulla in the Eocene and recent lemuriformes 

 is primitive for all primates is the presumably primitive form of the tympanic ring in these animals, 

 the association of this primitive ring with the relatively primitive position and course of the internal 

 carotid artery (see above), the retention of the stapedial branch, the apparently primitive unexpanded 

 condition of the brain, the prevailingly primitive dentition, etc. It might be objected that by the Law 

 of Irreversibility of Evolution if the ring was originally exposed and was then covered up it would be 

 impossible for Evolution to reverse itself and uncover the ring again. But such an interpretation of the 

 Law of Irreversibility would require proof. 



' Fonsyth Major, 1899, p. 987. 



