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Show 240 PROF. HUXLEY ON THE CRANIAL AND [Apr. 6, dation around and upon which the other parts are built, and reaches its adult condition early. Moreover it answers to one of the most important parts of the central nervous system, the base of the brain. It is therefore eminently fitted to furnish a relatively fixed unit of measurement and standard of position, to which the dimensions and the position of the other parts of the head and face, with the teeth, can be referred. In order to obtain such a standard, a median line is drawn in the bisected skull, from the hinder edge of the basioccipital bone to the junction between the prsesphenoid and the ethmoid in the base of the skull. I call this line the basicranial axis; and its value is taken as 100. The measurements of the other parts of the skull can then be expressed in terms of 100, and their development, irrespectively of the absolute size of the animal, becomes apparent. Sectional diagrams of different skulls, in which the basicranial axis has the same absolute length, show not only the different proportions of corresponding parts, but bring to light the relative depth, length, and inclination of the palate. This method of procedure is a little troublesome at first; but practice makes it easy, and the results are very satisfactory. When, as often happens, the skull under examination cannot be bisected, a sufficiently close approximation to the true length of the basicranial axis may be obtained by taking the distance along the median line of the base of the skull from the posterior edge of the basioccipital bone to a point opposite the middle of the distance between the optic and the ethmoidal foramina. This point always lies a little behind the posterior extremity of the vomer. 3. I will illustrate the method which I have described by comparing the skull of a common Fox with the skull of an animal which died in the Zoological Society's Gardens, and came to m e labelled " Canis azarce, South America." It corresponds very closely with the skulls also assigned to Canis azarce by D e Blainville (' Osteographie,' Canis, pl. iv.) and by Burmeister (' Erlauterungen zur Fauna Brasiliens'). In their actual dimensions and in their general form these two skulls are very similar, except that the zygomatic arch of the European is stronger and more sharply arched than that of the South- American animal, and that the longitudinal contour of the face is straighter in the Fox, in consequence of a slight convexity of the interorbital and posterior nasal regions in C. azarce. The ramus of the mandible of C. azarce is somewhat deeper at the level of the last molar tooth, and its coronoid process is less high and less inclined backwards, while the ventral contour is more sinuous. Other minor differences will be obvious on comparison of the figures. |