102 THE GROWTH OF GROUPS 



The most noticeable differences among the three are 

 to be seen on the ventral aspect and are exhibited by the 

 palatine foramina and the molar teeth, which are both 

 shown white in the illustration. In the skull C the 

 palatine foramen extends up to an imaginary line joining 

 the first molar teeth. In A and B it falls far short of 

 such a line. This is an important difference, since it is 

 constant : any mole-rat is in one state or the other as 

 regards this foramen. It is the custom to regard this 

 character as generic. A and B would be placed by taxo- 

 nomists in the genus Nesokia, C in the genus Gunomys. 

 This is not the only differential character between the 

 genera, but it is the most noticeable one. It may be 

 mentioned in passing that the distinction between the 

 two groups has long been recognized and is undisputed. 

 The two kinds occupy for the most part separate areas. 

 Nesokia is found in the north-western parts of India, 

 Gunomys in the southern and eastern parts. It is unusual 

 to find the two living side by side, as at Amritzar. The 

 size of the teeth, as seen especially in the breadth and 

 solidity of the molars, is also considered to be generic, for 

 it is the rule that rats with short palatine foramina have 

 large teeth. The skull B is, however, an exception to 

 this rule : it has the foramen of a Nesokia and the molars 

 of a Gunomys. 



Let us now examine the upper surfaces of the skulls. 

 A differs from the other two in being smaller as a whole, 

 in being broader in proportion to its length, and also in 

 the fact that the occipital condyles are invisible, owing to 

 the vertical position of the posterior surface of the skull. 

 A and B are much alike and differ from C in respect to 

 the proportionate length of the nasal bones (shortness of 



