PEIMATES FROM MADAGASCAR. 147 



may have had its share in accelerating the process of brain-degeneration which had 

 already set in 1 * 



Postorbital Wall. — With the narrowing of the postorbital frontal region has 

 probably come the almost complete disappearance of the postorbital septum. In the 

 various families of Monkeys the frontal postorbital region is generally so broad and 

 the orbits so comparatively small that the space to be filled in by the septum between 

 the brain-case and the orbital bar is only a small fraction of the circumference of the 

 eyeball — that is to say, a quite short outgrowth of the malar and frontal bones is 

 sufficient to bridge across the vacant space between the orbital bar and the brain-case. 

 The accompanying text-figure (41), which represents a transverse (nearly horizontal) 

 section through the orbit of Chiropotes, will show how small the actual septum 

 may be. 



In the case of the Malagasy Lemuroids which we are considering the narrowing of 

 the skull has carried the frontal, orbito-sphenoid, and alisphenoid far away from the 



Test-fig. 41. 



post- 

 orbital 

 seplutn 



Transverse (nearly horizontal) section through the orbit of CJiirojwtes. 



postorbital bar, so that supposing a septum did exist in a position analogous to that 

 which it occupies in the Apes, it would have to wall in a space which may be expressed 

 as equivalent to from 70 to 90 per cent, of the transverse diameter of the orbit. That 

 a more or less completely ossified septum did exist, however, in the ancestors of the 

 Indrisidse seems probable, for there appear to be distinct traces of it both on the outer, 

 upper, and occasionally inner edges of the posterior orbital contour f. Attention has 

 been called, in the case of Archceolemur, Mesopropithecus, and Palceopropitliecibs, to 



* I am aware that this theory B^ould imply the transmission to offspring of a character acquired during 

 the life of the individual ; but it has yet to be satisfactorily shown that a cause acting duiing long geological 

 ages may not be efficient where experiments extending over a comparatively few generations give purely 

 negative results. 



t In certain skulls of Archceolemur edwardsi there is a raised line on the inner wall of the orbit in exactly 

 the position where the septum would abut on the skull-wall were it continuous behind the eyeball. 



VOL. XVIII. — PAET II. No. 12. — May, 1908. x 



