176 



GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



In Mesoprojyithecus the whole region of the bulla is essentially the same as in Propithecus. The 

 foramen laceriim medium was closed and the canal for the art. promontorii ran along the outer margin 

 of the cochlea. (Fig. 67.) 



Indris is closely allied to Propithecus and no doubt agrees with it in the course of the carotid as it 

 does in the form and relations of the bulla. The inside of the brain-cavity of Indris as beautifully figured 

 by Edwards and Grandidier (1875, PI. xxxv, fig. 1) is likewise essentially similar to that of Propithecus. 

 The narrow slit marked c ("trou carotidien") is represented also in the dried Propithecus skull and appears 

 to be merely a remnant of the space between the petrosal and the alisphenoid. Although no distinct 

 foramina for the carotid lateral to the sella are shown in the figure, very probably in Indris the true caro- 

 tid foramina (for the art. promontorii) were even smaller than they are in Propithecus, and the main 

 supply for the cerebral arteries was drawn from the vertebral arteries as in Lemur. The venous sulcus 

 in which runs the stapedial branch, on the laterobasal corner of the brain-cavity, is clearly figured (op. 

 cit., PI. xxxv, fig. 2). 



In Lichanotus (Avahi) a British Museum specimen shows the anterointernal process of the bulla cover- 

 ing the region of the foramen lacerum medium; the Eustachian opening, foramen ovale and stylomastoid 

 are located as in Propithecus; a foramen at the posterior tip of the bulla is apparently the posterior carotid 

 foramen. 



In Palceopropithecus the collective characters of the skull, dentition and lower jaw prove that we 

 have to do with an aberrant specialization of the 7ndm-type, as will be shown in a later part of this work. 

 The whole basicranial region has been markedly altered from the Indris type, perhaps in correlation with 

 the great increase in size of the areas of origin and attachment of the masseter, internal pterygoid and 

 digastric muscles. The pterygoid plate of the alisphenoid no longer extends to the bulla, the bulla has 

 become flattened, the thick membrane that lies external to the tympanic annulus has become irregularly 

 ossified, so as to obstruct the opening of the external meatus. With all these changes it is not surprising 

 that the foramina in the region of the bulla should be correspondingly modified. Hence, taking the 

 normal Indrisidse as a guide, I conceive the long oval opening that lies posteroexternal to the pterygoid 

 process of the alisphenoid to be the foramen ovale and not the "foramen lacerum medium" as Standing 

 has named it (1908, p. 81). More or less confluent with the foramen ovale in some skulls, hut distinct 

 from it in others, is the Eustachian passage, which, as observed by Standing (pp. 81, 82) leads backward 

 into the shallow outer chamber of the bulla. A small opening between the foramen ovale and the Eusta- 

 chian foramen, which is present in the skull figured in Plate x, figure 1, of Standing's memoir, is of doubt- 

 ful character, but appears to represent a remnant of the space which once separated the outer expansion 

 of the bulla from the entoglenoid process of the squamosal, i. e., of the true foramen lacerum medium. 

 The foramen lacerum posterius, as in Indrisidse, is represented by two foramina which are here separated 

 by a considerable interval. The posterior carotid foramen was not located, but if the present interpre- 

 tation of the other foramina be correct it seems highly probable that, as in other Indrisidse, the whole 

 entocarotid was reduced, entering perhaps through some inconspicuous foramen (in the posterior part 

 of the bulla), which has hitherto escaped notice, and that the cerebra drew their arterial supply mainly 

 from the vertebral arteries. (Fig. 67.) 



ARCH^OLEMURINiE 



A skull of Archceolemur platyrhinus, No. 834 in the British Museum collection, shows beautifully 

 the interior of the bulla. It closely resembles that of Propithecus, save that it is wider. The canal for 

 the arteria promontcwii on the surface of the cochlea is very prominent and leads to the anterior end 



