HOOK-LIPPED OR BLACK RHINOCEROS 637 
regions, have in addition to the dentine and enamel a thick 
layer of cement which enters to an important degree into 
the composition of the teeth. Such teeth represent the 
highest specialization in rhinoceroses, and have long crowns 
in which the folds are united so as to enclose the cement 
layer as islands surrounded by enamel. Rhinoceroses are, 
without doubt, long-lived forms, but little data, however, 
are available upon which to base an estimate of the length 
of life of an individual in its native state. As they are not 
known to breed in captivity, practically nothing is known 
regarding the length of the period of gestation. But one 
young is produced at a birth. In body size the female is 
but little inferior to the male. The mammae are two in 
number. 
The extinct forms of rhinoceroses are very numerous, 
many different genera being represented throughout North 
America, Eurasia, and Africa, but so numerous have been 
the lines of divergence that it is quite impossible to trace 
back through the maze of forms any of the modern genera. 
The most ancient genera were contemporaneous in the 
Oligocene in both Eurasia and North America, but in the 
latter country they died out early in the Pliocene. In 
Eurasia the family persisted to the present time, and the 
modern Asiatic forms were evolved there during the Pliocene 
and Pleistocene. Africa, no doubt, also played an important 
part as a field of rhinoceros evolution, but, owing to the 
almost complete absence of fossil-bearing deposits in that 
continent, this is chiefly a matter of conjecture. The black 
rhinoceros has been reported by Scott from the Pliocene of 
Natal, and two other fossil species are described by Pomel 
in the Pleistocene of Algeria. A more significant discovery, 
