GREGORY: NOTHARCTVS, AN AMERICAN E(^CENE PRIMATE 



211 



(/i) two venous sulci opening into postglenoid foramen (Stehlin), but with certain differences in 



the course of one of them ; 

 (i) Whole course of internal carotid and its branches as in Lemur; 



Origin of the Lemurida; 



Dr. Stehhn (1912, p. 1294) concludes that the Adapidte are definitely excluded from further evolution 

 in the direction of the Lemuridae by the following decisive characters: 



(1) The early coalescence of the opposite rami of the mandible. 



(2) The ossification of the " annulus membrane." 



(3) The complication of the sinus hypotympanicus. 



(4) The complication of p^, which has two external cusps. 



(5) The construction of the incisors and canines. 



Of these, numbers 1, 4, 5 have already been discussed, the writer maintaining that numbers 1 and 5 

 are primitive characters of the Adapinse and partly degenerate and specialized characters in the Lemuridse; 

 with regard to number 4 it has been argued (see p. 135) first that the primitive Adapis sciureus probably 

 had a much more primitive p^ than the typical Adapis, and that, in view of the obvious degeneration of 

 the dentition in other characters, it would not be surprising if the ancestral Lemurida? had a second external 

 cusp on p'. 



With regard to characters numbers 2 and 3 it has already been noted that the "annulus membrane" 

 is not a true tympanic membrane, but an infolded surface of the bulla where it has grown around the 

 tympanic ring. It will be recalled^ that the whole sinus hypotympanicus or cavity of the bulla is merely 

 a diverticulum of the tubo-tympanal canal, which has either sunk into, and then become surrounded by, 

 the periotic or has acquired in some mammals an independent ossification in its own wall (entotympanic) 

 Originally the hypotympanic sinus was entirely medial both to the true tympanic cavity, with which it 

 communicated through the "pneumatic foramen," and to the tympanic annulus. Becoming greatly 

 inflated the bulla grew ventrad to the tympanic ring and finally concealed it entirely from the ventral 

 surface, at the same time the expanding bulla gained contact with the entoglenoid region of the squamosal 

 and with the ectotympanic plate of the alisphenoid. In surrounding the tympanic annulus the membran- 

 ous cavity of the bulla gave off diverticula, which, although differently developed in Adapis pai'isiensis 

 (Stehlin, 1912, p. 1208) and Adapis magnus (idem., p. 1254), in both cases nearly cover the tympanic 

 annulus with the infolded dorsal walls of the diverticula. 



The writer infers that a progressive enlargement of the pneumatic foramen, and concomitant retro- 

 gressive withdrawal or resorption of the osseous folds surrounding this foramen would finally free the 

 annulus almost entirely from the folds in question and at the same time so expand the diverticula that 

 they would become lost in the primary hypotympanic sinus. According to this interpretation the con- 

 ditions in the tympanic region of the modern Lemur represent a further advance, in the direction of 

 simplification, upon the conditions observed in the Eocene Adapinse, while according to Dr. Stehlin's 

 interpretation (1912, p. 1215) the Eocene genera (Adapis, Leptadapis) are more specialized in this region 

 than the modern Lemurida^. 



The writer's interpretation assumes only that the great size of the pneumatic foramen in lemurs 



^ Kampen, P. N. van. 1905, Die Tympanalgegend des Silugetierschadels, pp. 337-340. 

 - Op. cit., p. :363. 



