HAPLODONTIDiE— DISCUSSION OF AFFINITIES. 553 



coming between Castoridee and Sciurida, with close relationship on tire one 

 hand with Castor, and on the other with Arctomys. 



My views of the position in the Rodent scries, and relative rank in the 

 scale, of the Ilaplodont type rest upon an examination of the whole structure 

 of the animal. I do not find that any one has hitherto examined — at any rate, 

 given an account of — the viscera, or even the skeleton, excepting the skull and 

 leg-bones; our knowledge having thus far rested upon these portions of I he 

 bony frame-work, the teeth, and the*cxternal characters. lam, consequently, 

 enabled to add many new particulars to substantiate the position here taken. 

 Detailed descriptions are offered beyond under head of the species; here I 

 shall simply advert to some of the leading points involved. 



The skull of Haplodontidc? is strongly and unmistakably Sciuromorphie, 

 not only in its general structure, but in many ultimate details. In fact, it 

 resembles in superficial aspect the skull of certain typical Sciurines more 

 closely than some of these resemble each other. For example, no one who 

 compares the skull of Haplodon with that of Arctomys can fail to be struck, 

 as Dr. Peters was, with their close general resemblance. In comparison with 

 Sciurus, or even with such a Marmot-like form as Cynomys, the skull of Arc- 

 tomys is seen to be much more massive, much more depressed, broader behind, 

 and with a straightness and mutual perpendicularity of various planes, all of 

 which features would require Jittle exaggeration to match those of Haplodon. 

 Even the shape of the angle of the mandible, peculiar to Haplodon, is really 

 approached in Arctomys, where further twisting of the already oblique plate 

 of the descending ramus would bring it into the nearly horizontal plane 

 which it occupies in Haplodon. The most prominent difference is the total 

 absence of postoibital processes in Haplodon, and their full development in 

 Arctomys as in other Sciuridce. It may be fairly questioned, however, whether 

 the presence of these processes is more than a character of the family Sciu- 

 ridce' itself; for they are lacking* in the three other families referable to 

 Sciuromorpha. The preponderance of recent genera and species of Sciurid<c 

 may have unconsciously led us to attach too great importance to this feature. 

 Obviously, the fact that the family Sciuridce at present contains many genera, 

 while the other three Sciuromorph families have but one genus apiece, is no 

 argument for the making of postorbital processes a requisite for any series of 

 Sciurines of higher value than a family. The argument is the other way, in 



*Mr. Alston calls them "obsolete" in Jnomaluridw. 



