GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



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the membranous hypotympanic cavity. Beneath and to the outer side of this t)ulla lay the tympanic 

 ring (bearing the tympanic membrane) in a nearly horizontal position. 



Forsyth Major (1899, pp. 987) described the ontogenetic development of the bulla of the Malagasy 

 lemurs as follows: 



I 



Fig. .59. Norma basalis of skull of Adapis pari,sini.s'is. Fig. 60. Norma hasalis of skull of Adapis 



Enlarged. After Stehlin. magnus var. Iccuhurdii. Natural size. After 



Stehlin. 



In the youngest stage available to me for examination, the foetus of a Chiromys, there is no trace of an osseous 

 bulla; the completely ossified annulus lies almost horizontally underneath the periotic. In a second stage (Lepidolemur) 

 ossification begins to be developed from the lower sharp margin of the periotic, which adjoins the annulus. In a third 

 stage {Lepidolemur) this outgrowth appears increased, and has a shell-like shape, with the concavity turned outward; 

 the annulus is gradually being uplifted by it. In a fourth stage {Lemur rubriventer) the shell-like ossification is still 

 more increased, and begins to cover the median part of the annulus; and this state of things is still more increased in 

 the fifth {Lepidolemur) and sixth stage {Avahis laniger), with the result that first the median part, and eventually the 

 remainder of the annulus becomes invisible when viewed from below, being shut by the periotic. In the adult (as will 

 be seen by the skull of an adult Lemur ruhrirentcr which I exhibit) the annulus is represented by a bonj- ring — the size 

 is scarcely larger than in the youngest stages — which hangs freely in the tympanic cavity, being coalesced with the 

 squamosum only in one part, viz. anteriorly to the stylomastoid foramen. Ontogeny thus teaches us that the annulus of 

 the adult is not a secondarily detached part of the l)ulla." 



