138 Zoological Society. 



gently curved, and rather suddenly pointed at the end. In the re- 

 cumbent condition they incline a little towards each other, just over- 

 hanging the extremity of the glans, and bear some resemblance to the 

 pointed lower incisors of some small Rodent. The glans itself appears 

 tripartite at the extremity, there being a deep fissure running the whole 

 length of its under surface, and just at the extremity another on each 

 side : at the meeting-point of the fissures is the urethral orifice. Just 

 behind the origin of the bony stylets the presence of a small ossicle 

 can be distinctly felt within the substance of the glans. 



A very remarkable peculiarity in this little animal is, that amidst 

 the long white hairs which clothe the lower part of the foot is a small 

 sharp horny spike, situated just below the base of the middle toe, as if 

 it were intended to enter the ground, and thus prevent the animal from 

 slipping when it alights. This I have reason to believe is not generally 

 known, although it must I think be alluded to by Dr. Shaw in his Ge- 

 neral Zoology, since he there remarks, " There is also a very small spur 

 or back-toe, with its corresponding claw : " and subsequently adds, 

 " nor does any vestige of it appear in the figure given by Dr. Pallas of 

 the skeleton." This may well be, since it is simply a cutaneous deve- 

 lopment, having no connection with the skeleton whatever. I have 

 looked at the specimens of the Jerboa in the British Museum, but in 

 consequence of their being dried and mounted, the little appendage, 

 which is concealed by the hair, was not to be perceived ; but in the 

 Alactaga, as well as the same circumstances would permit, I could see 

 that a little horny process existed, but was rough and blunt. 



In the dissection of an animal whose only mode of progression con- 

 sists of leaping with the hinder extremities, and which differs from 

 the other jumping Mammalia in the circumstance, that in the position 

 of rest the extremity only of the metatarsus is applied to the ground, 

 the muscles of the leg may be expected to afford some points of in- 

 terest. The most striking of these are, that none of the muscles situ- 

 ated upon the tibia remain fleshy for more than about half the length 

 of that bone, each terminating in a long tendon ; and that upon the 

 foot itself there are no muscles whatever, the actions of the flexors 

 of the toes being relieved by a strong ligament, which arises from the 

 os calcis, and divides into five, giving one to the middle toe, two small 

 sesamoid bones being developed in it ; and two divisions to each of the 

 other toes, the index and the annularis, each of which has also its 

 sesamoid bones, those furthest from the axis of the foot being rather 

 largely developed, extending some distance over the sides of the arti- 

 culation. The ligament near its origin contains three little supernu- 

 merary bones, one on the outer, two on the inner side ; the latter are 

 grooved for the passage of the tendon of the flexor perforans. On 

 the homology of this tendon I have next to remark. It might very 

 naturally be expected, that in animals having no thumb on the hinder 

 extremity, and in which the fibula is in great part wanting, the flexor 

 longus pollicis, which in man has its origin in the fibula, would be 

 either much reduced or absent ; but so far from such being the case, 

 it will be seen, on reference to any work on the comparative anatomy 

 of the muscular system, that this muscle exists, and that its tendon 



