141 



December 27th, 1836. 

 Richard Owen, Esq., in the Chair. 



The remainder of M. F. Cuvier's Paper on the Jerboas and Ger- 

 b'dlas was read. 



M. Cu\4er commences this memoir with observing that his atten- 

 tion has been particularly directed to the Rodentia, with a view of 

 arriving at a natural classification of the numerous species composing 

 that order, among which considerable confusion had hitherto pre- 

 vailed, particularly in the genera Dipus and Gerbillus, the relations 

 of which to other allied groups have been but very imperfectly un- 

 derstood by previous writers. 



The species included in the genus Dipus have been formed by 

 M. Lichtenstein into three divisions, which are distinguished by the 

 absence and number of rudimentary toes upon the hind feet. In the 

 first section are placed those with three toes, all perfectly formed ; in 

 the second, those with four, one of which is rudimentary ; and in the 

 third, those with five, two of these being rudimentary. M. Cuvier 

 states that he is unacquainted with the second division of M. Lich- 

 tenstein, but in the examination of the species belonging to the first, 

 in addition to the absence of rudimentary toes, he finds they are also 

 distinguished from those of the third by the forin of the teeth, and 

 the osteological characters of the head. These points of difference 

 he considers of sufficient importance to justify his making a distinct 

 genus for the Jerboas with five toes, adopting the name Allactaga, 

 given by Pallas to a species, as the common generic appellation. 



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

 of the AUactagas, 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 AUactagas, and of one only in the Jerboas. The incisors 

 of the AUactagas 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 AUactagas 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- 



