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Show 6 MR. W. H. FLOWER ON THE [Jan.14, utility to the zoologist; and this will be more especially the case when the part in question is one so imperishable, so easy of examination, and affording indications so clear and capable of ready definition and description, as the base of the skull. In order not to extend this communication to too great length, or over too great a variety of subjects, I propose to limit m y observations on the present occasion mainly to the terrestrial or fissipedal Carnivora, and only to those genera now existing. M y reason for this last restriction is, that it is only in these that we have the opportunity of thoroughly working out all the important points of structural modification throughout the system, and thus definitely assigning their position; and from these only can we hope to establish any correlation between the structure of the hard and imperishable parts and the viscera. When such a correlation has been established, then the examination of the fragmentary remains of the extinct forms can be made with much greater advantage, and the work of tracing the stages by which the present condition of the order has come into being can be approached with more probability of a satisfactory result. The region to which attention will now be especially directed is the posterior part of the base of the cranium, the most conspicuous feature in which, in all Carnivora, is the auditory bulla ; and it is mainly the characters of this bulla, and the structures immediately surrounding it, which will be described in the principal genera of the order. Following Mr. Turner's example, I will first take one of the extreme forms of existing terrestrial Carnivores, the Bear. Figures 1 and 2 (pp. 7 & 8) are taken from the skull of a not quite adult Ursus ferox in the Museum of the Royal College of Surgeons (No. 4016). The auditory bulla is comparatively little inflated. It consists of a single bone, readily detached from the cranium in skulls of young animals. Its form is more or less triangular, being broad and nearly straight at the inner edge, and prolonged outwards into the much produced floor of the external auditory meatus (m.a). Its greatest prominence is along the inner border; from this it gradually slopes away towards the meatus. Near the hinder part of the inner edge is a considerable circular foramen (cur), which pierces the bone obliquely, leading to a canal which runs forwards in an arched direction, in its inner wall. This is the carotid canal. In old Bears the entrance is partly concealed by the prominent lip of the basioccipital, which abuts against the inner edge of the bulla; and by the growth of this and of the paroccipital process it becomes almost included in the deep fossa leading to the foramen lacerum posticum (I). Anteriorly the carotid canal of the bulla ends close to the inner side of the groove for the eustachian tube; and the artery quitting it takes a sudden turn upwards and backwards and enters the cranium through the foramen lacerum medium. When a section is made through the auditory bulla (see fio-. 2, p. 8) it is seen to be a simple thin-walled bony capsule, imperfect above where it fits on to the petrosal and squamosal bones, and prolonged |
