530 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxi. 



seen only in the specimens from Borneo and Banka. The only avail- 

 able skull of a Xyctlceinis from Java is youno-, and it can not be poi^i- 

 tively stated whether the temporal ridges would meet in the middle 

 line or whether they would develop heavy parallel ridges as in the 

 Bornean form. The ridges on it, however, look more as they do in 

 3^oung of the species where a sagittal crest is formed than they resemble 

 those in young Bornean examples. Blate XIII shows the progressive 

 development of the two styles of temporal ridges. 



There is a progressive increase in size of the skulls with increasing 

 age, which is show^n in both the Bornean and Malay Peninsula series. 

 A full set of teeth is no criterion that an animal is fully adult, and 

 onlv in ver}^ old age do the teeth show much wear. In eomparing 

 any two species, it is necessary to select individuals of the same age. 

 (See table of measurements, p. 537, and Plate XIII.) 



As noted by most writers, the number of upper incisors in the Slow 

 Lemurs varies between two and four, the usual view being that the 

 smaller outer pair are dropped in old age. An examination of the 

 present material shows that the Bornean Slow Lennu- never has but two 

 upper incisors. Examination of a very young skull. Cat. No. 142240 

 U.S.N.M., probal)ly a newborn individual, shows only two upper 

 incisors and no possible place for the small lateral pair of incisors. A 

 very young Sumatran skull, Cat. No. 141142 U.S.N.M., has four upper 

 incisors, two adult Sumatran skulls have four, one adult two, and one 

 adult three. A young Malayan skull, Cat. No. 84390 U.S.N.M., has 

 three; all the other mainland skulls and the one from the Natunas show 

 four upper incisors. In Anderson's Catalogue of Mammalia in the 

 Indian Museum" it appears that the mun))er of upper incisors is vari- 

 able in the mainland species. In the Javan skull the premaxilhe are 

 wanting and nothing can be told about the upper incisors. Ander- 

 son '' and Milne-Edwards' state in Javan skulls the number maybe 

 either two, three, or four. It looks as if the Bornean form and possi- 

 bly the one from Banka possess only two upper incisors, while in all 

 the other forms four upper incisors are found, always in the young 

 and often in the adults. If more material should show that this is the 

 case and that it is correlated witii the uiuuiited temporal ridges, the 

 Bornean and Bancan Xyctiojbi would form a distinct subgeiuis. For 

 a tabulation of the number of upper incisors see table of measure- 

 ments, page 537. 



DESCRIPTION OF THE SPECIES OF ST.OW LEMURS. 



In the following account of the species I have made use of binomial 

 names only as there does not appear to l)e sufficient material to work 

 out intergradation satisfactorily. In general it maybe stated that the 



"Parti, pp. 95-97. 



^Anderson, Western Yunnan Expedition, Zool. Results, 1879, p. 105. 



'Milne-Edwards, Nouv. Archiv. du Museum, Bull., Ill, pp. 10, 11. 



