DICROSTONYX 



143 



on each side, formed by the squamosals alone, the lateral processes 

 of the supraoccipital being short. Interparietal of normal shape, 

 wider than long ; in some forms it articulates with the squamosals, 

 but in others it is separated from those bones by narrow tongues 

 of the parietals which pass back on either side to articulate with 

 the supraoccipital. The cheek-tooth rows diverge slightly behind ; 



Fig. 70. — Dicrostonyx henseli Hinton. 



Dorsal, ventral, and lateral views (enlarged) of the type skull from the late 

 Pleistocene fissure deposit at Ightham, near Sevenoaks, Kent (B.M., 

 No. M.l 1,803, Geol. Dept.). The small outline is a natural-sized 

 representation of the skull in dorsal view. 



the alveolar capsules, in consequence of the great height of the 

 teeth, rise up in the floor of the orbit and in the sphenorbital 

 fissure ; in correlation with this and with the breadth of the 

 teeth, the presphenoid is reduced in most species to a slender 

 bar. Palatal structure differing from that of Microtus chiefly in 

 the extension further forwards of the mesopterj'goid fossa and in 

 the shortness and free termination of the post-palatal median 

 septum, which is here represented merely by a short median 



