60 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



extent of the symphysis menti. In the form of the incisors, the more contracted 

 palate, the great extent of the symphysis menti, and in fact in most of the points 

 of dissimilarity, between the skull of the present animal and that of Mus decu- 

 manus, here pointed out, it will be perceived, there is an approach made to the 



Arvicolidce. 



The dimensions of the skull (so far as an imperfect specimen will allow of 



their being taken) are as follows : — 



In Lines. 



Distance between front of incisors, (upper jaw) and the first molar tooth 8 



Longitudinal extent of the three molars on either side, taken tggether . 4^ 



7i 



of incisive foramina ........ 4£ 



Width between orbits . . 



Length of ramus of lower jaw ....... 



Length of nasal bones 



o n 



Fig. 3, c, Plate 33, represents the molar teeth of the upper jaw. Fig. 3, d, 



those of the upper jaw. 



" This rat was caught at Bahia Blanca where the plains of Patagonia begin 

 to blend into the more fertile region of the Pampas. It lived in holes amongst 

 the tussocks of rushes, on the borders of a small, still, brook ; in its manner of 

 diving and aquatic habits it closely resembled the English water-rat, (Arvicola 



amphibia.)" — D. 



When at Paris I examined what I believe to be the original Mus Braziliensis, 



since the specimen was labelled " Rat de Brazil St. Hilaire, 1818." It agrees 

 perfectly with the present animal excepting in being rather smaller, the length 

 from the nose to the tail being 7 inches and 4 lines— the length of the tail is 7 

 inches 9 lines, and that of the tarsus is 1 inch 1 1 lines ; this difference in the 

 length of the body may arise from difference of age, or even of sex. In the Paris 

 Museum I saw what appeared to me to be a variety of the same species in which 



the under parts of the body are white. 



I have been minute in my description of the Mus Braziliensis, since it is 

 confounded by Desmarest, Fischer and Lesson with the Rat troisieme or Rat 

 Angouya of Azara, which I believe to be a very different animal. The descrip- 

 tion given by the authors just mentioned are taken from Azara, who gives the 

 following characters to distinguish the Rat Angouya : " Du museau a la queue, et 

 sur les c6tes du corps tout est brun-cannelle, parceque les poils ont une petite 

 pointe cannelle ; puis, ils sont obscurs et enfin blanc vers las peau. Toute la partie 

 inferieure de l'animal est blanchatre, plus claire sous la tete, et plus foncee entre 

 les jambes de devant ; le pelage est doux, tres-serre, et le poil, qui est a la racine 

 de l'oreille, cache le conduit de celle-ci." 



