seen only in the specimens from Borneo and Banka. The only available skull of a Nycticebus from Java is young, and it can not be positively 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 young of the species where a sagittal crest is formed than they resemble those in young Bornean examples. Plate 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 shown in both the Bornean and Malay Peninsula series. A full set of teeth is no criterion that an animal is fully adult, and only in very old age do the teeth show much wear. In comparing 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 Lemur never has but two upper incisors. Examination of a very young skull, Cat. No. 142240, U. S. N. M., probably 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[1] it appears that the number of upper incisors is variable in the mainland species. In the Javan skull the premaxillæ are wanting and nothing can be told about the upper incisors. Anderson[2] and Milne-Edwards[3] state in Javan skulls the number may be either two, three, or four. It looks as if the Bornean form and possibly 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 with the ununited temporal ridges, the Bornean and Bancan Nycticebi would form a distinct subgenus. For a tabulation of the number of upper incisors see table of measurements, page 537.
DESCRIPTION OF THE SPECIES OF SLOW LEMURS.
In the following account of the species I have made use of binomial names only as there does not appear to be sufficient material to work out intergradation satisfactorily. In general it may be stated that the
