54 MR. E. T. BENNETT ON THE CHINCHILLID. 
apparent capacity of the cranium, and present externally, in consequence of their mag- 
nitude and the tenuity of their parietes, the appearance of three large vesicular pro- 
tuberances on each side of the cranium. Of these the superior, placed immediately 
mesiad of the upper margin of the external meatus,( which is a large open cavity, pene- 
trating deeply from above downwards,) is nearly hemispherical: the posterior, situated 
behind the meatus, is oblong, with its long diameter from above downwards: and the 
inferior, which with its fellow occupies nearly the whole inferior surface of the cranial 
cavity, is pyriform with its long diameter from before backwards, and its uarrowest 
portion pointing backwards and outwards. At the point of junction of the posterior 
and inferior of these protuberances, which have but a slight appearance of separation 
from each other, the styloid process passes down, closely applied and firmly anchylosed 
to their substance. 
In Lagotis, on the contrary, the tympanic cells have little increased development, 
and none of the vesicular appearance. Those of the upper surface of the cranium are 
placed at some distance mesiad of the margins of the meatus externi, and are flat and 
scarcely distinguishable: the posterior are long, narrow, and flattened: and the inferior 
bear no comparison to the size of the same parts in Chinchilla, although resembling 
them in shape. The external meatus is formed nearly in the same manner; but the 
styloid process is free from any attachment to the outer parietes of the cells. The whole 
tympanic apparatus of Lagotis does not equal one third of its proportional size in Chin- 
chilla. 
In both animals the rami of the lower jaw posterior to their union are remarkably 
thick and strong, and its angular plates thin and delicate: the coronoid process is but 
little developed, and that of the angle is much prolonged, especially in Chinchilla. It 
terminates in a point, between which and the condyle there occurs a broad, deep, semi- 
lunar excision. The condyle is small and longitudinal, and the glenoid cavity super- 
ficial, admitting of great freedom of motion in the antero-posterior direction. 
In both animals the number of cervical vertebre is, as usual, seven ; the dorsal and 
lumbar are together nineteen ; but unless a rib on each side has been lost in the prepa- 
ration of Lagotis, (and of this I can perceive no proof in the existence of an articular 
surface,) its dorsal vertebre are only twelve, while in Chinchilla they are certainly 
thirteen. Two anchylosed vertebre form the sacrum in each; but the number of caudal 
vertebre differs, Lagotis having twenty-seven, and Chinchilla only twenty-three. In 
both the atlas is broadly developed, and there is a considerable spinous process on the 
dentata ; but scarcely any elevation exists at this part on the other cervical vertebra. 
The spinous processes of the dorsal vertebra, from the third to the ninth inclusive, are 
much elongated, and directed backwards ; the tenth has the same direction. Those of 
the lumbar vertebre are directed forwards, and are remarkably strong and conspicuous 
on the three last, as well as on the two sacral, where these processes resume a vertical 
direction. The caudal vertebre, with the exception of the first seven, are long and 
