28 



Memoirs of the Indian Museum. 



[Voi. I, 



Dimensions of M. decumanus. 





Head and 

 body. 



Tail. 



Hind foot. 



Ear. 



Rough average 



22-6 



20-2 



415 



19-7 



Average large . . . . . . ... 



24 







• • 



Average small 



21 







•• 



Maximum 



27-4 



24-5 



4-8 



2-2 



Minimum . . . . . . . . .: 



19 



16-5 



3-8 



1-5 



Rough average percentage . . 



100 



89 



18 





Large Do. 



100 





19-5 





Small Do. 



100 





17 





Maximum percentage 



100 



100 



21 





Minimum Do. 



100 



80 



16 





The two largest rats met with, of a total body-length of 27 and 27*5 cm., were both 

 males, but the next two largest, 25 6 and 25, were both females. Though practically 

 every proportion of tail from 80 to 100 per cent, of total body-length was met with, 82 

 per cent, of the total fall within the limits 85 to 95 per. cent. The percentage length 

 of the hind foot is a rather important measurement as it is an extremely valuable 

 indication of the age of the specimen. The four rats which have as small a measure- 

 ment as 16 per cent., are all very large and old rats with a body-length of 275, 27, 257, 

 25 "25 cm. On the other hand the two with a maximum of 21 per cent, are barely full 

 grown, measuring only 19 and 19-5 cm. in body-length. With half-grown rats about 

 II ctn. in length, the foot is 28 to 30 per cent, of the body-length. 



Skull. — 'The most striking characteristic is the flatness of the antero-posterior 

 curve and the general appearance of squareness shown by the cranium. The upper cra- 

 nial surface, which in Mus rattus is pyriform, is here rather rectangular, the massive 

 supraorbital ridges running back almost straight to join a very well-defined occipital 

 ridge. The occipital bone is set obliquely instead of vertically, so that the condyles 

 project most posteriorly of any part of the occiput. The anterior edge of the zygoma- 

 tic plate sweeps boldly upwards and forwards to recurve to a deep emargination. This 

 is not very well shown in the figures as the anterior curve should be much bolder. 

 The nasals only just project beyond the gnathion or most prominent point of the 

 premaxilla, so that they are much less prominent than in Mus rattus. The interparietal 

 is generally three-sided with a well-marked anterior spike, but this may be suppressed 

 so that the bone is an oval as is normally found in the young skull, vide fig. 18. The 

 same variability of the posterior suture as regards its position relative to the occipital 



