RHINOCEROS. 599 



across the section, and those of one layer in an opposite direction 

 to that of the subjacent layer of fibres. 



Retzius notices the existence of radiate cells in both the 

 radical and the coronal cement of the Rhinoceros's grinder : they 

 were of various shapes, some round and a^ooth of an inch in diameter, 

 some angular, and others prolonged in the form of tubes ; all receive 

 numerous fine tubuli which are clustered around them. 



217. Succession. — The deciduous dentition of the genus Rhino- 

 ceros, is : — 



2-2 - 4-4 ^. 



incisors — : molars — : = 24. 



2-2 ' 4-4 



In the Sumatran Rhinoceros the small mid-incisors of the 

 lower jaw (PI. 138, fig. 15, d 1) are first shed and replaced by 

 a larger pair {i 1), which protrude beneath them; these are small 

 in comparison with the lateral pair, and are also shed before the 

 last true molar cuts the gum. The first true molar appears before 

 any of the deciduous set are shed, but the first milk-molar soon 

 yields place to the first premolar ; the second milk- molar gives 

 way to the second premolar, and about the same time the second 

 true molar advances into place. Next the deciduous outer incisors 

 (ib. d 2) their fang deeply excavated by the absorbent process 

 excited by the pressure of their large successors {i 2), are pushed 

 or broken out; the last deciduous molar is displaced, and the last 

 premolar rises above the gum about the same time that the last 

 true molar comes into place. Thus, notwithstanding the close 

 similarity of form and structure between the premolars and molars, 

 each division of the permanent masticatory series has its own 

 order and progression of development, and thereby manifests its 

 essential distinction. The last milk-molar is not more complex 

 than the last premolar which takes its place. 



218. PalcBotherium. — The vast hiatus, which, in the series of 

 existing Mammals, divides the Rhinoceros from the Tapir and this 

 from the Elephant, was once filled up by interesting transitional 

 species of anisodactyle Pachyderms which have long become extinct. 

 I shall briefly notice the leading features of the dentition of some 

 of those ancient forms of Mammalia, as indeed, this enduring part 



