400 Zoological Society. 



the common Squirrel appear very diiFerent 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 J3y which the external carotid artery emerges 

 from the cranium is very apparent, and I have not perceived it to 

 exist in any of the Sciurldce. 



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. 



