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 (H. s7valensis) 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 (4. 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 
