PEIMATE8 PROM MADAGASCAE. 75 



eventually obtained, this distinction appears to be no longer tenable, and all the 

 specimens are referred to the one species P. maximus. The average maximnm 

 length of the skull of Pal(Bo;pwpithecus maximus, measured from the lambdoidal 

 crest to the alveolar margin of the incisors, is 198 mm. (max. 216, min. 181*). 

 The average bi-zygomatic breadth for six skulls where both arcades are complete 

 measured 121 mm. (max. 135'5, min. 113). The ratio between the maximum 

 breadth and maximum length expressed as a percentage index is 61. Similar 

 measurements taken on a skull of Imlris give the number 59"4. The maximum 

 external breadth across the orbits in the case of Palceopropithecus is 98 mm., giving, 

 when compared with the maximum length of the skull, an index of 4.9*5 ; similar 

 measurements on Indris and Propithecus give indices of 61'5 and 63'6 respectively. 



The form and proportions of the individual bones composing the brain-case conform 

 in general to the condition observed in Indris and Propitheciis. 



Occi/pitcd Region. — The occiput is truncated and forms a plane at right angles to the 

 long axis of the skull. The foramen magnum is, generally speaking, circular in 

 outline, but varies much in size and shape, corresponding diameters showing a 

 discrepancy of 5 or even 7 mm. in different skulls. The occipital condyles are broad 

 and prominent, frequently in adult skulls projecting backwards beyond the lambdoidal 

 crest (Plate X.). The paroccipital processes are extraordinarily developed, their free 

 internal margins attaining in some cases a length of 20 mm. and extending downwards 

 and backwards nearly as far as the condyle itself. In its basal portion this process 

 follows along the margin of the occipito-temporal suture as a prominent ridge, in some 

 instances reaching the lambdoidal crest. On its outer edge it is fused to the strongly 

 developed mastoid process of the squamosal and also to the styloid process. The 

 prominent convexity for the lodgment of the central lobe of the cerebellum, which is a 

 marked feature of the occiput of the extant Indrisinse, is absent in Palceo^propitheciis, 

 this region of the skull-wall being much thickened and of porous texture. The 

 lambdoidal ridge is prominent in all but quite young skulls, and in old individuals 

 forms a sharp crest which descends along the mastoid process, an upper branch 

 passing above the external auditory meatus to join the upper edge of the high 

 zygom0,tic process of the squamosal. Where it is still possible to trace the lambdoidal 

 suture, the supraoccipital is seen to enter to but a small extent into the roof of the 

 cranium. In one young skull the supraoccipital occupies 18 per cent, of the distance 

 from the lambdoidal crest to the root of the nasals (text-fig. 9), while in Pro]iithecus 

 more than 25 per cent, of this distance is covered by the supraoccipital. 



Parietal Eegion. — The brain-case in the parietal region is narrow and depressed, 



* "When dimensions are given in tlie following description three numbers will generally be quoted, the first 

 being the average for a series of several skulls ; and foUoTving this in parentheses are the maximum and 

 minimum measurements noticed in any individual. 



VOL. xviiL— PART II. No. 3. — May, 1908. m 



