212 



Gregory: notharctus, an American eocene primate 



is secondary; it coincides completely with van Kampen's conclusion^ that originally the hypotympanic 

 sinus was sharply separated from the true tympanic sinus by a septum (formed by the osseous shell of 

 the hypotympanic cavity) and that, by the enlargement of the pneumatic foramen, this septum was 

 reduced, so that finally only vestiges of it remain {Tupaia, Lemuridse, most Canidse) or it vanishes en- 

 tirely (Rodentia, many ungulates, etc.)- 



Fig. 78. Auditory region of Adit pin, Xofhardus, Lemur. 



1. Notharctus oshorni. Amer. Mus. No. 1146B. Twice natural size. 



2. Ada-pis magnus. After Stehlin. Twice natural size. 



3. Lemur varius. Amer. Mus. No. 10424. Natural size. 



This well-grounded conclusion from comparative anatomical data thus supplies a probable explana- 

 tion of the fact that the tympanic ring lies freely within the cavity of the bulla instead of remaining 

 morphologically outside of that cavity as it should in a primitive lemur and as it actually does, except 

 to a very limited extent, in the Eocene Adapinse. 



' Op. cit., p. 339. 



