* THE EGYPTIAN JERBOA. 33 



evidently the same, and is characterized by the large 

 size of the cranium, the shortness of the muzzle, and, 

 above all, by the magnitude of the suborbital foramina. 

 The cranium of the Jerboa is distinguished by its great 

 breadth posteriorly, resulting from the enormous deve- 

 lopment of the tympanic bone, which extends beyond 

 the occipital posteriorly and laterally, as far as the zygo- 

 matic arch, which is by no means the case in the Alac- 

 tagas, where all the osseous parts of the ear are of mode- 

 rate dimensions. Another differential character between 

 the two genera is presented by the maxillary arch, which 

 circumscribes externally the suborbital foramina, and 

 which in the Alactagas may be said to be linear, pre- 

 senting a very limited surface for the attachment of 

 muscles. He then notes a difference in the relative 

 development of the jaws, the lower being comparatively 

 much shorter in the Alactagas than in the Jerboas, 

 Having described a new species of Alactaga, a native 

 of Barbary, under the name of Alactaga arundinis, M. 

 r. Cuvier proceeds to consider the characters and affi- 

 nities of the genera Gerbillus and Meriones, and enters 

 into a critical examination of all the species referred to 

 those genera, and comes to the conclusion that they have 

 a closer affinity with the true Muridce, than with the 

 Jerboas and Alactagas. Fig. 17 represents the skull 

 and teeth of Dipus hirtipes : a, skull, profile ; ft, the 

 same seen from above ; c, the same seen from below ; d, 

 c, the teeth. 



Fig. 18 represents the skull and teeth of Alactaga: 

 a and b, the cranium, one- third larger than the natural 

 size ; c and d, the teeth, five times enlarged. 



The Egyptian Jerboa {Dipus jEgyptius). 



In the true Jerboas the head is large, and not unlike 

 that of a rabbit in form ; the ears are long and somewhat 

 pointed ; the eyes are full and prominent ; the tail is 

 very long, cylindrical, and covered with short hair, 

 except at the extremity, which is tufted. The fur of 

 the body is soft and delicate ; the whiskers are long ; 

 the fore-feet are very small, and have four toes, and the 



