144 CLASS MAMMALIA. 



is nearly equal to that of the Rat. Its head is thick, and 

 large in proportion to the body ; but yet more elegant in 

 form than that of the Alactaga. The nose is smaller, and 

 the ears shorter and larger. The upper incisors, which are 

 generally white, are vertical, square, and divided in their 

 length by a groove, which runs through the middle. The 

 silken hairs of the mustachios are about six inches long. 

 The eyes are large, projecting, and lateral, and separated 

 from each other by about an inch and a half, and have a 

 brown iris. The ears are whitish at the base, the rest gray. 

 The body is rather elongated, and larger behind than be- 

 fore. The general colour of the skin is a clear fawn, and 

 varied with black and zigzag lines, which contrast agree- 

 ably with the beautiful white of the under part of the 

 body. 



Naturalists do not agree on the number of this animal's 

 toes. Besides the three toes armed with obtuse nails, Son- 

 nini says that he has observed near the heel, a kind of spur, 

 or rather a slight rudiment of a fourth toe. From this cir- 

 cumstance he regards the Alactaga, described by Samuel 

 Gmelin, as apparently belonging to the same species with 

 the Gerbo. Most authors, however, differ on this point with 

 Sonnini. Pallas gives five toes to the Mus Jaculus, and but 

 three to the Mas Sagitta. Edwards declares that he has 

 not been able to discover the little spurs above mentioned. 

 M. Olivier, and, above all, our illustrious author, coincide 

 with the opinion of Pallas. The only authors of Sonnini's 

 opinion are Buffon, who never saw either the Gerbo or Alac- 

 taga, and J. F. Gmelin, who confounds the two species, 

 under the name of Dipus Jaculus, while he admits as a 

 distinct species the Mus Sagitta of Pallas, which is un- 

 doubtedly the same animal as the Gerbo. 



We may as well add here some anatomical points of dis- 

 tinction observed by M. Desmarest. 



The tail of the Gerbo has six vertebrae less than that of 



