328 New Species of Aplodotttia. 



As a whole, the skull of the female is less heavy and massive 

 than that of the male, and the occipital crest is not so highly de- 

 veloped. The zygomatic arches are not so much bowed out- 

 ward, and the post-zygomatic notches are larger. In all these 

 points the females show an approach, though distant, to the 

 condition of the adult male of A. rufa. 



The study of a series of young, with which I am fortunately 

 provided, serves to bridge over very completely the gap left be- 

 tween the adult females of A. major and adults of A. rufa. 

 The evidence of this is found not only in the occipital region, in 

 the lesser development of the various crests and processes, and 

 in the post-zygomatic notch, but in the size and shape of the 

 zygomatic arches and the vacuities which they enclose ; in the 

 length and shape of the nasals ; in the length and ratios of the 

 fron to-parietal line and of the premaxillaries ; in the position of 

 the antorbital foramina, iu the ratio of the upper molar series to 

 the basilar-length — in fact in all the characters which serve to 

 distinguish the adults of the two species. 



The growth of the teeth in A. major has not kept pace with 

 that of the skull. They were doubtless large enough in A. rufa 

 to subserve the wants of the animal ; and since the increase in 

 size which the California species has undergone has not been 

 accompanied by change in food-habits, a corresponding enlarge- 

 ment of the teeth has not been necessary. Conversely, while 

 the grinding apparatus remains the same in both, the machinery 

 by which it is worked has come to be largely increased in size 

 and power. 



From the foregoing study of the development of the skull in 

 the present species, it appears that Aplodontia major was de- 

 rived from A. rufa or its immediate predecessors, and that it is 

 the most highly differentiated form yet known of the remarka- 

 ble, primitive Haplodont type of the Sciuromorph series. 



