164 



GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



. At the same time Dr. Major stated the important fact "that in the Tertiary Adapts the annulus 

 tympanicus is a free ring, independent of the bulla [but lying within it], absolutely as in the Malagasy 

 lemurs." 



If, as is commonly supposed, this ontogenetic process in lemurs gives a clue to the actual course of 

 evolution, it is evident that in all the Malagasy lemurs, in Adapis and Nothardus, the hypotympanic 

 shell of bone has extended outward beyond the tympanic ring so as to conceal it completely from the 

 ventral view. Continuing to grow laterally and anteriorly it gained contact with the pterygoid wing 



, ,. ,. , „ p , , • tig- t)2. iSorma basalis of skull of 



ric. Dl. iNorma basalis oi skull oi Adapis maqnus. ,, , , tit -.c^m^ 



, . 1- Aotharduscrassus. Anier. Mus. Ao. 12567. 



iNatural size. Alter btehlin. t.,-,,, t-. /t- t> • i x tut 



Middle Eocene (L pper Bndger), \S yoming. . 



Natural size. 



of the alisphenoid and with the adjacent entoglenoid process of the squamosal; posteriorly its expansion 

 was limited by the exoccipitals and internally by the basioccipital. 



The hypotympanic sinus still communicates with the true tympanic cavity by a more or less widely 

 open fissure, lying between the tympanic annulus and the auditory prominence and called the pneumatic 

 foramen. The sinus hypotympanicus, or cavity of the bulla, lying beneath the cochlea, an essential 

 organ of hearing, probably acts as a resonating chamber and possibly may have incidental advantages 

 in regulating the pressure on either side of the tympanic membrane. The tympanic annulus itself in all 



