ON ARCTOCEBUS CALABARENSIS 601 
grinding surfaces of the upper and lower molars appear to be at first 
sight, they are really arranged upon much the same scheme. 
Furthermore the transition from premolar to molar is effected in 
the same way: in the lower as in the upper molars —by the development 
of cusps which are rudimentary in the premolars, and by the 
appearance of an oblique ridge. 
The dentition of the genera Lords (Stenops), Nycticebus, Pero- 
aicticus, Otogale, Galago, and Otolicnus, resembles that of the Ang- 
wantibo in all essential particulars, but presents certain very 
interesting minor deviations. 
In Lorts the third premolar differs much from the first molar in 
both jaws; and the last molar of the upper jaw has its internal 
posterior cusp well developed, so that it is quadricuspid. The last 
molar of the lower jaw is quinquecuspid. The oblique ridges of both 
upper and lower molars are well developed; but, in the lower jaw, 
they do but just reach the anterior internal cusp. 
In Myctecebus javanicus the oblique ridges are well marked in the 
upper molars, sometimes less distinct in the lower ones. Out of four 
skulls, the inner posterior cusp of the hinder upper molar was obsolete 
in three, and very small in the fourth. In the lower jaw, the fifth 
cusp of the last molar was very small or obsolete in three, while in 
the fourth (the same as that which had the inner posterior cusp of the 
last upper molar developed) it was distinct. 
The dentition of Perodicticus potto (fig. 4), it is important to note, 
differs more from that of the Angwdantibo than either of the preceding. 
For the posterior upper molar is short and wide, so as to have a 
transversely elliptical crown, which has only two cusps, the posterior 
external, as well as the posterior internal cusp having disappeared. 
In addition to this, the hinder upper molar is set further out than the 
other molars, and the hinder lower molar has only four cusps. 
In all the species yet mentioned, as in the Angwantibo, the last 
premolar has but a short heel, and differs considerably from the first 
molar. But in Osogale pallida, Galago sennaarensis (fig. 5), G. maholl, 
G. allenti, Otolicnus garnetti/, and O. crassicaudatus the heel of the 
hinder premolar above and below becomes so large and cuspidate as 
to assume the form of a quadricuspidate molar. This is particularly 
well seen in Galago sennaarensis (fig. 5) and G. allenzt. In the latter 
species, even the second premolar acquires a great heel, so that the 
transition from canines to molars is quite insensibly graduated. 
In Galago minor, on the other hand, the third premolar is as 
different from the first molar as in the Angwantibo. 
