266 MOSTLY MAMMALS 



full complement of incisor-teeth consists of three pairs, it is 

 natural to suppose that one pair has been lost in the common 

 species. That such is really the case is demonstrated by 

 the extinct Siwalik hippopotamus (//. sivalensis) of the 

 Pliocene deposits of the outer ranges of the Himalaya. 

 Here between the two large tusks there are three pairs of 

 incisor-teeth, which differ from those of the common species 

 in being all of nearly equal size ; and if we were to 

 examine the upper jaw, we should find that in this also 

 there is the same number of teeth. In the presence of 

 these three pairs of incisors the Siwalik hippopotamus 

 resembles the pig, from which it departs less widely than 

 does the common species in that these teeth are relatively 

 smaller and also of nearly equal size. The Siwalik hippo- 

 potamus must accordingly be regarded as a less specialised 

 species than either of its living cousins ; and since, together 

 with an allied species from the Irrawady Valley known as 

 the Burmese hippopotamus (//. iravaticus)^ it is the oldest 

 representative of the genus, its generalised features are 

 precisely what evolutionary considerations would have led 

 us to expect. 



There is, however, yet another curious point in con- 

 nection with these teeth demanding a moment's notice. 

 From the evidence of the two species mentioned, it is 

 quite impossible to determine which of the three pairs of 

 lower incisors found in the Siwalik hippopotamus have 

 disappeared in the common species. Fortunately, however, 

 palaeontology here once more comes to our aid, showing 

 not only which pair has been lost, but how the loss was 

 brought about. From the gravels of the Narbada Valley 

 in Central India, which are probably intermediate in age 

 between the Pliocene deposits yielding remains of the 

 Siwalik hippopotamus and the brick-earths of our own 



