406 Zoological Society. 



any use need be made of such characters as these, when the groups 

 can be so well established upon characters more obvious and import- 

 ant in their nature. 



We frequently find groups which, though very extended as to the 

 number of species they contain, are much more limited in respect to 

 the varieties of structure they present than other groups appa- 

 rently of equal rank containing a much smaller number of species. 

 Such groups are of course always the most easy to isolate, but the 

 most difficult to subdivide : it is in these that we find the most con- 

 fusion existing, and the greatest variety of opinion among naturalists 

 as to the manner in which their subdivision should be effected. Ex- 

 cepting in the highest divisions, it is but of late years that naturalists 

 have at all appreciated the distinction between what are usually 

 termed "essential" and "adaptive" characters, of the former of 

 ■which, as we descend to the lower groups, not only is the existence, 

 but also the importance, much less easily recognized. 



The base of the cranium, as I before observed, is, from its having 

 less connexion than most parts of the bony framework with the pe- 

 culiar wants of the species, by far the most rich in such characters ; 

 among those which the foramina may afford, I must here dwell 

 rather particularly on the evidences of affinity afforded by the pre- 

 sence or absence of the ali-sphenoid canal, and also explain my rea- 

 sons for assigning it a new name. As will apjiear from the observa- 

 tions I have brought forward, it exists throughout the Rodentia, ex- 

 cepting the aberrant family of Hares ; it is wanting in the Marsupials 

 and Edentata; and among the Ungulate division, including the Ru- 

 minants and Pachydermata, the Artiodactyle division, including the 

 Ruminants and those Pachyderms which have the toes in even num- 

 ber, is constantly characterized by its absence ; while in the Perisso- 

 dactyla it is as constantly present. In the first edition of the ' Lemons 

 d'Anatomie Comparee,' the illustrious author only alludes to this canal 

 in a very vague manner ; and in the more recently published edition, 

 in which the osteology of the cranium is much more fully elaborated, 

 it is spoken of everywhere as being the vidian canal, — the existence 

 of a vidian canal being denied in those animals which do not happen 

 to possess it. From the time when 1 commenced the series of obser- 

 vations of which the present is an attempt to sum up the results, I 

 always felt inclined to the belief that the canal in question did not 

 correspond in situation to tiie vidian canal as known in Human 

 Anatomy, since this canal commences just at the root of the internal 

 pterygoid process, while that jjointed out as such in the work alluded 

 to is quite on the outside of the homologue of the outer one. Among 

 the rest, the Monkeys are spoken of as wanting the vidian canal ; but 

 on removing from the skull of a small monkey in my collection the 

 whole of the posterior portion, and the temporal bones with auditory 

 bullae, the posterior apertures of the vidian canals became very ap- 

 parent, and fine bristles passed readily through them into the orbits ; 

 and ia other skulls belonging to the Quadrumanous order, provided 

 that those portions of the upper maxillary bone which originally con- 

 stitute the alveoli of the hinder molars do not rise high enough to 



