70 MR. 8. J. A. SALTER ON THE 
the posterior nares, the position of the incisor teeth are all differ- 
ent ; but these characters and other minor ones will be better 
appreciated by referring to the specimens themselves and the 
illustrations. 
I have not thought it worth while to figure or describe com- 
paratively the skull of Mus decumanus. I may mention, however, 
that it is very distinct from that of the new rat ; indeed it is more 
like the skull of Mus rattus. The common Brown Rat’s skull is 
rather longer and slenderer than either of the others; it is nar- 
rower across the cerebral region, and does not there bulge out in so 
rounded a form, but is more oblong. The two ridges which pass 
backwards from the frontal bone, at the top of the zygomatic 
fosse, scarcely extend to the parietal bones in the new rat; in 
Mus rattus they diverge and bow out in a crescentic form over the 
parietal bones, whereas in Mus deewmanus they pass back sharp, 
rigid and parallel. The foramen magnum occipitale is even more 
extended laterally than in Mus rattus : it is not so deep vertically, 
and has not the creseentic notch in the centre of its upper outline. 
In the skull of Mus decumanus there is a little process projecting 
backwards from the front angle of the zygomatic fosse; I have 
found it in every skull of the Brown Rat I have examined : it does 
not exist in either of the others. 
Blasius, in his ‘Fauna of the Mammalia of Central Europe,’ 
gives an admirable figure of the skull of Mus deewmanus (fig. 171, 
page 310): it is critically correct, and has all the distinctive 
characters which mark the cranium of this rat.* 
I am fully aware that too much importance should not be 
attached to observations made on single specimens; and I am 
aware, too, that allowance should be made for the possibilities of 
individual variety. I regret that I have been unable to multiply 
my specimens; but it is difficult to obtain many, either of the 
Black Rat or the Snake-Rat. Ihave reason, however, to think that 
the different kinds of rats are not liable among themselves to 
any very marked individual varieties in the anatomical characters 
of their crania, T have had opportunities of examining enormous 
numbers of the common Brown Rat’s skull. The crania have been 
all exactly alike: Blasius’s figure might have been copied from 
any one of them. Again, the differences between the two skulls 
T have contrasted are of such importance, and so grave, that they 
seem inconsistent with mere variety: indeed I am not aware that 
* Fauna der Wirbelthiere Deutschlands &., Naturgeschichte der Siuge- 
thiere, von J. H. Blasius. 1857. 
