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 hasicranial 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, irre- 

 spectively of the absolute size of the animal, becomes apparent. 

 Sectional diagrams of different skulls, in which the hasicranial 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 httle 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 

 hasicranial 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 com- 

 paring the skull of a common Fox with the skull of an animal which 

 died in the Zoological Society's Gardens, and came to me labelled 

 " Canis azarce, South America." It corresponds very closely with 

 the skulls also assigned to Canis azara by De Blainville (' Osteo- 

 graphie,' Canis, pi. iv.) and by Burmeister (' Eriauteruugen 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 in- 

 clined backwards, while the ventral contour is more sinuous. Other 

 minor differences will be obvious on comparison of the figures. 



