CHAP. V] THE CRANIA OF THE SIMIIDAE (PRIMATES) 229 



the angle is a large one (varying from about 120° to 140°). 

 Turning to the human skull, we find that the mean value is 

 rather greater (the variations ranging from about 137° to 157°). 

 But if now we turn again to the lower Anthropoidea, to the lowlier 

 primate forms and mammalian orders, we find that the angle is 

 very much smaller, and has much more nearly the value of a right 



Fig. 152. Cranium of a Dog (Carnivora, Canidae) bisected in the median 

 sagittal plane ; to shew the two sections into which the cranio-facial axis has been 

 conventionally divided. The line Op. B. represents the foramen magnum; B. Pr. 

 the " middle base," and Pr. N. the " anterior base." 



angle ; this is very evident in the Cynocephalous monkeys, or in 

 Carnivora (cf. Figs. 150 c and 152). 



We arrive thus at the conclusion that in the evolution of the 

 form of the skull important changes have taken place in the 



N 



Opv 



B 



Fig. 153. This and the three succeeding figures represent the component parts 

 of the cranial axis in the several stages which mark the path of evolution of the 

 human type (Fig. 156) from the generalized mammalian type (Fig. 153, with which 

 cf. Fig. 152). In the latter (Fig. 152) the line B. Pr. represents the comparatively 

 stable middle portion, extending from the basion (cf. Chapter x) to the prosphenion, 

 or most anterior point of the sphenoid bone. B. Op. represents the plane of the 

 foramen magnum; and Pr. N., the line from the prosphenion to the nasion, 

 represents conventionally the plane of the cribriform fossa. These indications 

 apply to the whole series of figures (150 to 159 inclusive). 



relations of the anterior and posterior basal portions respectively, 

 to the intermediate, centrally-placed part. We see that if the latter 

 be regarded as comparatively fixed, the anterior or ethmoidal part 

 at one end, and the posterior or foraminal portion at the other, 

 vary from animal to animal in such a way as to enable one to 



