ARVICOLA 415 



1847. Hypudceus spelceus Cuvier, Giebel, Fauna der Vorwclt, 1, p. 88 



(nomen nudum). 

 1869. Arvicola amphibixs Boyd Dawkins, Q.J.G.S., 25, p. 194 ; Sanford, 



ibid., 26, p. 124, 1870; Blackmore and Alston, P.Z.S., 1874, 



p. 462 (in part). 

 1890. Microfiis amphibius Woodward and Sherborn, Cat. Brit. Foss. 



Vert., p. 365; Newton, Q.J.G.S., 50, 1894, p. 196. 

 1910. Arvicola abbotti Hinton, Ann. Mag. N.H., [8], 6, p. 34; Proc. 



Geol. Assoc, 21, p. 494. 



Type. — B.M., No. 11804; adult skull, nearly complete, from 

 the collection of W. J. Lewis Abbott; presented by Sir H. H. 

 Howorth, K.C.S.I., F.R.S. 



Type locality and horizon . — A deposit filling fissures in the Hythe 

 Beds (Lower Greensand) at Ightham near Sevenoaks, Kent, 

 England. Late Pleistocene. 



Range in time and space. — Known only from the Late Pleisto- 

 cene of Britain, where it occurs commonly in the later cavern and 

 fissure deposits. 



Characters. — A large member of the A. scherman group; with 

 skull as large as in A. amphibius, the condylo-basal length 

 reaching 41 mm. in adults; the limb skeleton relatively small 

 and light. 



Skull (Fig. 108) characterized by its straightened and 

 protruding incisors and sloping occiput; both characters, the 

 result of extreme fossorial specialization, far more pronounced 

 than in any living member of the A. scherman group. Occiput, in 

 adults, sloping forwards conspicuously, the interparietal reduced 

 and its posterior margin boldly convex instead of being nearly 

 straight; lambdoidal crest markedly sinuous, and the ridge for 

 the ligamentum nuchm prominent; basioccipital tilted upwards 

 and backwards, forming a marked angle in front with the basi- 

 sphenoid. In younger specimens the occiput is more nearly 

 vertical, the interparietal less reduced, with straighter posterior 

 border, and the basioccipital more nearly horizontal. Post- 

 orbital squamosal crests somewhat more extensive than in A. 

 amphibius, but much less salient. Anterior palatal foramina very 

 small in correlation with the relatively broader incisors. Ptery- 

 goid fossae, for origin of pterygoidexis internus muscles, more 

 extensive, the postero-lateral palatal pits wider and the mesoptery- 

 goid fossa narrower correspondingly. Auditory bullae about 

 as in A. amphibius, but with smaller external apertures and a 

 more copious filling of spongy bone. 



Mandible with very small angular processes ; incisor ascend- 

 ing nearly to the condyle, its growing base marked by a strong 

 hump upon the outer surface of the condylar process, which is 

 sharply inflected above the hump. 



Incisor teeth relatively broad. Cheek-teeth very Ught, but 

 similar to those of A. amphibius and other species in pattern. 



For measurements, see table at end of volume. 



