PEIMATES FEOM MA.DAGASCAE. 89 



divides into two curved ridges, which spread widely to form the posterior edge of the 

 raised frontal convexity, and are continued with slight interruption to form the outer 

 edge of the postorbital bar. 



In adult animals the median suture is closed along the whole length, in the parietal, 

 frontal, and nasal regions. The spheno-parietal suture is similar in length and 

 position to that of Propithecus. The outlines of the squamosal and the extent to 

 which it rises up along the skull-wall closely resemble the condition observed in the 

 modern genera. 



The Frontal Begion. — The form of the frontal and orbital regions of Ilesoprojnthecus 

 sharply differentiates the genus from the extant Indrisinee, while closely allying it 

 with the Archgeolemurs. Different skulls show considerable diversity as to the 

 elevation and amount of convexity of tlie frontal region between and behind 

 the orbits. In the case of the specimen figured on Plate XXI., this part of the skull 

 is raised so as to be actually higher than the central parietal region, but in every 

 instance the interorbital part of the frontal bone is much more sharply curved down- 

 wards than is the case with the extant members of the suborder, thus giving to the 

 profile a very Ape-like aspect. 



A frontal sinus is present, comparable in extent with that found in Indris and 

 PropUhecus. A noticeable feature of Mesop^'opithecus, and one which we have seen 

 to exist in all the recently extinct subfossil Lemuroids, is the narrowing in of the 

 postorbital region of the frontal bone. The extent of this constriction does not, 

 however, in Mesopropithecus appear to be so great as materially to affect the frontal 

 lobes of the brain. There is, in fact, a very close resemblance between the brains of 

 Mesopropithecus and Propithecus *. 



The Orbital Begion. — Compared with the existing Indrisinee the orbits of Mesopro- 

 pithecus are proportionately smaller and more forwardly directed, the postorbital bar 

 is stouter and more distinctly triangular in section, and the malar region while not 

 so deep and strong as in Palceopropithecus is considerably more developed than in 

 Propithecus or Indris. In the case of the last two genera the orbit has so great a 

 lateral extension and the zygoma so flattened a curvature that when the skull is 

 viewed from the front the zygomatic arch is hardly visible. In Mesopropithecus, on 

 the contrary, the zygomata spread widely away from the brain-case beyond the external 

 orbital rim, in this respect again closely resembling Archceolemur (text-fig. 16). 



The development of the malar region in conjunction with the elevated frontal and 

 steep facial angle must have given to Mesopropithecus a more Monkey-like face than 

 that even of Archceolemur. 



* See Appendix by Elliot Smith, infra, p. 168. 



