1901.J LEMUR MOKGOZ AND L. EUBRIVENTER. 267 



Finally, the palatal sinus covers completely the orbital maxillary 

 plate also, and constitutes for itself the whole inflated bottom of 

 the orbit (PI. XXII. fig. 4). As has already been mentioned, the 

 sinus expands also on the basis cranii ; the most posterior part 

 of the palatal, the processus pyramidalis, becomes inflated, and 

 the choanie are considerably narrowed by the swollen pars perpen- 

 dicularis (PL XXII. fig. 7, pp)- 



In very old specimens a partial absorption of the roof of this 

 sinus takes place (PI. XXII. fig. 5). 



I have found this very curious palatal sinus in all the specimens 

 of Lemur rubriventer examined, eight in the Zoological Department 

 of the Natural History Museum, seventeen in the Leyden Museum, 

 and six specimens forming the remainder of my collections made in 

 Madagascar. 



It is one of the characteristics of pneumatic cavities in Mammalia, 

 that they often vary greatly within the same genus, so as to offer 

 excellent specific characters for systematic purposes, when, as is 

 the case in the present and in the before described species (Lemur 

 mongoz), the change in the skull is such that it becomes apparent 

 without any dissection being necessary. The skull of Lemur 

 rubriventer can at once be recognized by the unique feature of its 

 bulla-shaped palatal sinus. 



As to its particular function, it certainly has not the one to 

 supply space for the teeth. Conversely, a section of The maxillary 

 sinus in this and other species shows that the latter apparently 

 has amongst its functions to favour or protect the development of 

 the two posterior premolars ; and in the L. momjoz before described, 

 the peculiar posterior sinus affords a similar protection to the two 

 anterior true molars. But the palatal sinus of L. rubriventer 

 certainly interferes with the development of the last true molar, 

 for its floor grows over the corresponding part of the maxilla at 

 the very time when the tooth is in the state of germ. The fact 

 that L. rubriventer is the one species of the genus which has 

 the smallest and most reduced m. 3 (see PI. XXII, fig. 7) is a con- 

 firmation of the above. 



The distribution of these pneumatic cavities over the whole of 

 the family (Lemuridae), and their various degree of development in 

 different groups, gives a clue as to their principal function when 

 they ai-e greatly developed. 



In those Lemurs which are characterized by their sluggish 

 movements (Loris, Nycticebus, Perodicticus), the pneumatic cavities 

 are small. In the nearly related Galagos, known for their agility, 

 the sinuses around the cavum nasale are equally reduced ; but the 

 pneumatic cavities of the mastoid region evidently act as a com- 

 pensation. In Malagasy Lemurs inflations of the mastoid region 

 are an exceptional occurrence and, when present, never greatly 

 developed, except in " GMrogaleus trichotis." The maximum of 

 development of aerial sinuses, almost exclusively of those starting 

 from the cavum nasale, occurs in the larger forms of Malagasy 

 liemurs, all of them excellent climbers and acrobats. 



