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 beg 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 
