1864. ] AND DENTITION OF THE LEMURID, 631 
the periotic. In all of them also, a single opening represents both 
the sphenoidal fissure and the foramen rotundum. 
As regards the dentition, in all four genera the first upper premolar 
is longer* than the second. The third upper premolar has only one 
large external cusp, while the first and second molars have each two 
well-developed subequal external ones; these two teeth, moreover, 
have always the oblique ridge running from the postero-external to 
the antero-internal cusp. There is a difference also between the 
third lower premolar and the first lower molar similar to that existing 
between the same teeth of the upper jaw. 
In the Slow Lemurs also the gall-bladder has not its fundus di- 
rected towards the back, and the dorsal and lumbar vertebre are both 
very numerous, the former being never less than fourteen, nor the 
latter less than seven in number,—twenty-one being thus the lowest 
number of both dorsal and lumbar vertebree, taken together, which is 
found in any Slow Lemur. 
It is an interesting fact, that, as far as concerns the skull and den- 
tition, the Asiatic Nycticebus far more resembles the African Pero- 
dicticus than it does its oriental neighbour, Loris. It does so in the 
breadth between the orbits, in the non-prolongation forwards of the 
preemaxilla, in the length of the first upper premolar, in the smaller 
size of the last molar, both above and below, in the large size of the 
upper incisors, and in the shortness of the bony palate. 
Loris. Nycticebus. 
On the other hand, the African Arctocebus differs more as regards 
its dentition (as Professor Huxley has pointed out) from its geogra- 
phical fellow, Perodicticus, than it does from Nycticebus; while it 
has a certain resemblance to the Asiatic Loris. Thus it agrees with 
the latter in the large size of the last molar, both above and below, 
in the smallness of the upper incisors, and in a certain prolongation 
forwards of the nasal spine of the preemaxilla. Arctocebus, however, 
agrees with Perodicticus in having a smaller number of dorsal and 
lumbar vertebre than Loris and Nycticebus ; and it differs from all 
the other Slow Lemurs in the small vertical extent of its first upper 
premolar. 
In the typical genus Lemur the mastoidal region of the periotic is 
not enlarged and inflated, thus differing from that of the Galagos 
and Slow Lemurs. The length of the muzzle, from the anterior ex- 
tremity of the preemaxilla to the front margin of the orbit, exceeds 
the antero-posterior extent of the anterior opening of the latter. The 
preemaxilla is plainly visible when the skull is viewed laterally. The 
most anterior point of the hinder margin of the palate reaches, or 
nearly so, to the anterior end of the last molar. A malar foramen is 
* This is very slightly so in Arctocebus: see ante, p. 321, fig. 3. 
