RATS AND PLAGUE. 165 
is not strictly the case: yet it may be said that while the grooves 
of Nesokia are most distinct, those of the rats are very ill- 
defined and visible with the help of a strong lens only. 
Another readily observed difference between the two lies 
in the form of the molars, more especially of the upper series. 
Those of Nesokia are divided transversely into laminae; those 
of Mus sinuously into cusps: these features are shown most 
clearly when the teeth are worn. 
The body of Gunomys is stout, that of Mus slender; varia- 
tions that are again strongly emphasized in the skulls, that of 
the former being short, broad and deep, robust and solid in 
construction while the latter is elongate, slender, shallow and 
of a more delicate appearance: in Gunomys the nasal bones fall 
short of, or never project beyond, the front surfaces of the 
incisors: in Mus the nasals are so elongated that if the skull is 
viewed from above the incisors are completely hidden. Again, 
viewed laterally, the zygomatic arch of Mus is almost in a 
plane with the alveolar edge of theupper molars, that of Gunomys 
falls far short of this. 
A further notable difference which has not before been 
remarked on may be seen on the outer surfaces of the ascending 
rami of the jaw bone. Where on the base of attachment of 
the masseter muscle we find in Mus merely a slight tubercle or 
protuberance, there occurs in Gunomys a distinct upward- 
pointing spine having between it and the surface of the ramus 
so deep a gap that the spur appears almost as defined as the 
coronal point or condyle. 
Mus validus of which the skull most nearly approaches in 
form and structure that of Gunomys has this tubercle rather 
more developed than have other rats and in the bamboo-rats 
(Rhizomys Spp.) it is even more exaggerated than in the Nesokia 
group. 
The only comparison Mus griseiventer needs for our 
purpose is with Mus decumanus. In both the dorsal pelage is 
harsh and wiry but not essentially spiny and the abdomen 
grey or drab coloured. The upper colouring of the former is 
somewhat warmer, the tail is dark throughout and slightly 
R. A. Soc., No. 57, 1910, 
