Zoological Society. 395 



** We know," observes M. Cuvier, "^that the three principal toes 

 of the Allactagas, as well as the three only toes of the Jerboas, are 

 articulated to a single metatarsal bone, and that the two rudimentary- 

 toes of the first genus have each their metatarsal bone ; whence it 

 results that the penultimate segment of the foot is composed of three 

 bones in the Allactagas, and of one only in the Jerboas. The incisors 

 of the Allactagas are simple, whilst those in the upper-jaw of the 

 Jerboas are divided longitudinally by a furrow. The molars of the 

 latter genus are complicated in form, and but little resemble those of 

 the former. They are four in number in the upper-jaw, and three in 

 the lower, but the first in the upper is a small rudimentary tooth, 

 which probably disappears in aged individuals." 



The structure of the grinding teeth is then described in detail, and 

 illustrated by drawings which accompanied the paper. 



" The general structure of the head of the Allactagas and Jerboas 

 is 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 distin- 

 guished by its great breadth posteriorly, resulting from the enormous 

 development of the tympanic bone, which extends beyond the occi- 

 pital posteriorly and laterally as far as the zygomatic arch, which 

 is by no means the case in the Allactagas, where all the osseous parts 

 of the ear are of moderate dimensions. Another differential character 

 between the two genera, is presented by the maxillary arch, which 

 circumscribes externally the suborbital foramina, and which, in the 

 Allactagas, may be said to be linear, and presenting a very limited 

 surface for the attachment of muscles. Lastly, we may note a dif- 

 ference in the relative development of the jaws, the lower being com- 

 paratively much shorter in the Allactagas than in the Jerboas" 



The author then proceeds to describe a new species of Allactaga, 

 a native of Barbary, for which he proposes the name of A . arundinib. 

 Its length from the origin of the tail to the end of the muzzle, 5 inches ; 

 length of the tail, 5 inches and 2 or 3 lines; of the ears, 1 inch; length 

 of the tarsi from the heel to the extremity of the toes, 22 lines. All 

 the upper parts of the body are of a beautiful greyish yellow, with 

 yellowish sides and tail of the same colour, terminated by a tuft of a 

 blackish brown at its origin, and white at the extremity. The sides 

 of the cheek, the ventral surface of the body, and the internal limbs 

 are white ; large brown moustaches adorn the sides of the muzzle. 

 The incisors are white and entire, the ears almost naked. 



M. Cuvier next proceeds to consider the characters and affinities 

 of the genera Gerbillus and Meriones, and enters into a critical ex- 

 amination of all the species referred to that group. To these he adds 

 another species, the habits of which he details, and describes at length 

 under the name of G. Burtoni. The species which he thus includes 

 are, 1st, G. Egyptiacus, syn. Dipus Gerbillus, Meriones quadrima- 

 culatus, Ehrenberg ; 2nd, Gerbillus pyramidum, syn. Dipus pyramidum 

 Geoff., Meriones robustus Riipp. ; 3rd, G. pygargus, syn. Meriones 

 Gerbillus, Riipp. ; 4th, G. Nidicus, syn. Dipus Nidicus, Hardwicke ; 

 5£h, G. Africanus, syn. Meriones Schlegclii Smutz., G. Afra Gray; 



3 E2 



