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the common Squirrel appear very different from each other, there is 
in fact but little to distinguish, so far as these peculiarities are con- 
cerned, between the families to which they respectively belong. In 
the latter animal, and indeed throughout the family, we find the fol- 
lowing arrangement: the foramen ovale is a large round hole, within 
the edge of which open the posterior orifices both of the true ali- 
sphenoid canal and the canalis ali-sphenoideus externus, and also of 
a canal which only penetrates the substance of the basi-sphenoid 
bone, and meets its fellow from the opposite side. The foramina 
lacerum anterius and posterius are each of small size; I cannot per- 
ceive any distinct canalis caroticus. In the Rat the canalis ali- 
sphenoideus externus does not exist, its place being marked by a 
rather indistinct groove in the bone; the true ali-sphenoid canal is 
present, and its posterior opening is some distance anterior to the 
foramen ovale ; the foramen entering the substance of the basi-sphe- 
noid also exists, but is situated some distance behind the foramen 
ovale; from the posterior corner of the external pterygoid process 
there is continued a little bridge of bone, which arches completely 
over the foramen ovale: there is no canalis caroticus, a groove only 
representing it. But in specimens that I have dissected for the pur- 
pose, I have noticed that the external carotid artery actually enters 
the cranium through a canal in the posterior part of the tympanic 
bone, from which it emerges above, and after passing within the 
cranium for a short distance, passes out again through the long fissure 
that separates the anterior side of the tympanic bone both from the 
ali-sphenoid and the squamous bones; it then passes through the 
little bridge that crosses the foramen ovale, and then through the 
ali-sphenoid canal, after which it, as usual, meets with the second 
branch of the fifth pair of nerves, and accompanies it through the 
infra-orbital foramen to the upper lip. But the chief differences here 
pointed out between the Rat and the Squirrel seem only to consist 
in the extension backwards, in the latter, of the ali-sphenoid canal 
to the foramen ovale, and the presence or absence of the lamina that 
encloses the canalis ali-sphenoideus externus. Some genera of Rats 
(as Cricetus, Cricetomys, Hapalotis, Hydromys and others) present in 
these respects the same characters as the Squirrels, in some of the 
larger species of which we even see a very slender arch of bone just 
before the foramen ovale. However, in all those genera of Rats 
alluded to, the fissure by which the externa! carotid artery emerges 
from the cranium is very apparent, and I have not perceived it to 
exist in any of the Sciuride. 
In the Edentate order, which, though so limited in the number of 
species, is far from being so in the variety of its forms, the foramina 
present characters which will connect together those forms which 
other and more important characters show to be nearly allied. In 
the Armadillos the optic foramen is small and distinct; the foramen 
rotundum has coalesced with the foramen spheno-orbitarium; the 
foramen ovale is a distinct, roundish aperture: there is usually a 
distinct canalis caroticus, but in the Dasypus sexcinctus it is only en- 
closed at the anterior part; and in one specimen that I have seen, 
