162 



GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



by Stehlin (1912, p. 1203-1215). The general plan of construction of the tympanic region in these genera 

 is that common to the existing Malagasy lemurs and has been very thoroughly investigated by Forsyth 

 Major (1899), van Kampen, Stehlin and others. In all these forms the bulla consists of a great ly expanded 

 bubble or shell of bone surrounding a diverticulum (sinus hypotympanicus) of the true tympanic cavity 

 which in turn is a dilatation of the tubo-tympanal duct ; the bony shell in question apparently does not 



Fig. 58. Norma basalis of skull of Noiharcfus oshorni. Amer. Miis. No. 11466. Middle Eocene (Lower Bridger), 

 Wyoming. Twice natural size. 



arise from a separate center like the os bullae (entotympanic) of many mammals, but is derived solely 

 from the periotic and represents perhaps what was once merely a rim of the peri otic, overlapping the 

 membranous hypotympanic cavity; the beginnings of such a rim are shown in certain insectivores, as 

 suggested by van Kampen (see also Fig. 66). This periotic shell or rim doubtless gradually grew over 



