70 



MB. S. J. A. SA.LTEE 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 

 fossae, 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 decumanus 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 crescentic 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 fossae ; 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 decumanus (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. I have 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'a figure might have been copied from 

 any one of them. Again, the differences between the two skulls 

 f have contrasted are of such importance, and so grave, that they 

 seem inconsistent with mere variety: indeed T am not aware that 



* Fauna dor Wirbolthioro Doulsolilandl ko. t Naturgosohiohte dor Siiugo- 

 \)urrr; urn J. II. Hla*iua. 1857. 



