10 Memoirs of the Indian Museum. [Vol. I, 



A skull which has a maximum cranial breadth of 50 per cent, of the total skull-length 

 and an upper molar series of 22 per cent, of the skull-length is that of a rat which is 

 cutting or has just cut its third upper molar , has attained only 50 to 60 per cent, of its full 

 body-length, and possesses a skull that is only about 70 per cent, of its full length. 



The relative proportions in the adult vary of course according to the species, 

 being greater in the broad-skulled Nesokia than in the long-skulled M. rattus, but it 

 may roughly be laid down that in the adult the corresponding percentage measurements 

 are- 37 % and 17 % against 50 % and 22 %. In M. decumanus the cranial breadth sinks 

 to 33'6%, while the molar series is as low as 14-9%, probably owing to the fact that 

 the series from which the measurements were made were rather specially selected as 

 full grown. This is borne out by the series of maximum sized M. rattus, where these 

 measurements fall to 35 58% and 16-24% respectively. These figures apparently do 

 not hold good for some of the Nesokice, owing to the excessive breadth of the cranium 

 and the relatively large size of the teeth ; but the data on which to form a conclusion 

 are at present deficient. Apart from actual measurements there are general character- 

 istics of a young skull sufficiently well marked to enable one at once to form an opinion 

 as to its age. The globularity of the cranium is very distinct, and gives the posterior 

 aspect a very characteristic shape, as is very well shown in the plates accompanying 

 this paper. The occipital, instead of being a sloping or perpendicular plate, is a 

 segment of a sphere, and the part of the skull which projects most posteriorly is the 

 centre of the occiput, instead of being the occipital ridge or the condyles according 

 as the occiput is vertical or sloping. Again, the bony ridges for muscular attach- 

 ments such as the occipital and supraorbital are hardly developed at all, so that, 

 instead of forming bold contour lines which may in the adult give the cranium a 

 rectangular character rather than a globular one, they do not break the smooth 

 globularity of the cranium. 



In its undeveloped condition the zygoma tends to slope forwards and downwards 

 instead of being boldly perpendicular. This forward slope has already been referred 

 to in the case of Mus blanfordi. It is distinctly shown in Mus rattus, occasionally in 

 Nesokia bengalensis, and only in one out of several young M. decumanus. My 

 single specimen of the young Nesokia bandicota shows it distinctly, and the doubtful 

 young Kilakarai skull shows it very clearly indeed, the slope being over 30° from the 

 perpendicular. 



The characters of immature teeth will be dealt with when discussing the different 

 species, as the extra cusps and ridges or rudiments of them, which are so well shown in 

 a young tooth, vary so much in the different forms. 



The fur in young rats is soft, thick and mole-like, with dense grey underfur. The 

 longer hairs, the projecting tips of which determine the colour of the mature rat, are 

 very sparse and hardly break the general surface of the fur, so that the colour tends 

 to be grey more than brown. This is particularly so on the underparts, which are 

 coloured almost entirely by the underfur, so that the belly is dark grey with hardly any 

 lightening of colour. This applies to the young of all the rats I have examined. 



