Mr. BROOKES on a new Genus of the Order Rodentia. 99 
two-thirds of its length, by the ossification of the interosseous 
ligament. The toes are four in number, terminated by small 
: claws, and the skeleton exhibits not the slightest rudiment of a 
thumb. In the Jerboa the scapula is still more delicate; the 
acromion, though slender, does not exceed one-third of the 
length of the spine; the os brachii is weak, and its deltoid pro- 
cess a simple but strong tubercle ; the radius is twice as long as 
the arm-bone, and there is a marked rudiment of a thumb, 
which is visible even in the living animal. © 
The general appearance of the pelvis in Lagostomus is de- 
licate ; it is comparatively narrow, and is wider in its trans- 
verse than in its sacro-pubal diameter : its position is extremely 
vertical, whence it appears incapable of affording much sup- 
port to the abdominal viscera when the animal assumes the 
upright position. The ossa pubis are but little produced ; their 
symphysis is slender and much elongated, and the obturator 
foramen is consequently enormously large. nage 
The ilia are long and narrow, and their crista, which is blunt, 
is little expanded. The ossa femoris are straight, strong, and 
without ridges; they are furnished, like those of the rabbit, 
squirrel, and some other animals, with three trochanters, the 
ordinary trochanter major and trochanter minor, with a tro- 
chanter externus, situated a little below the larger process. 
The tibia and fibula are nearly half as long again as the femur ; 
the fibula is complete, extending downwards, and forming the 
malleolus externus. The os calcis is strong, and elongated 
backwards: the metatarsal bones are three; they are strong ; 
the middle one is not quite one-half of the length of the tibia : 
at the tarsal extremity of the outer one there is a small some- 
what curved and obtusely-pointed tubercular elongation directed © 
backwards, as though it were intended as a fulcrum to give 
additional security to the foot in leaping. A similar formation 
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