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 speciahzation 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 countr}^ 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, 



