376 MICROTIN^ 



second inner valley persisting as in M. irdermedius, not reduced by 

 insulation of its internal portion as in M. plioccenicus ; posterior 

 salient angle on each side small, m^ in adults with three sub- 

 stantially closed triangles, four outer and five inner salient angles ; 

 its third outer valley not reduced by the insulation of its internal 

 portion, but persisting as in M. majori; a vestigial fourth outer 

 valley often present; third outer salient angle without a " prism- 

 fold " ; fourth outer and fifth inner salient angles becoming 

 obsolete in later stages of wear. Lower incisor wholly lingual 

 to the roots of ?«2. 



The foregoing diagnosis may be supplemented usefully by the 

 following notes on the scanty remains by which the species is at 

 present represented in collections. Although Forsyth Major 

 (op. cit., p. 105) records the species as occurring in the Norwich 

 Crag, I have been unable to find any remains from that deposit 

 definitely assignable to M. newtoni in the British Museum, the 

 Museum of Practical Geology, Norwich Museum, or any other 

 collection to which I have had access.^ The only British remains 

 known to me were obtained at East Runton, Norfolk, where, 

 however, they occur apparently in two quite distinct deposits. 

 Those in the British Museum, including the type of the species, 

 were collected by Mr. Savin from a bed of " shelly crag 30 yards 

 from the cliff to the north of the gangway" at East Runton; 

 this deposit is usually correlated with the Weybourne Crag. In 

 the Museum of Practical Geology is a small collection of vole 

 remains obtained by the late Mr. Clement Reid, F.R.S., from a 

 bed of "clay-gravel" at East Runton. Mr. Reid described ^ this 

 bed as a laminated clay, belonging to the middle or estuarine 

 division of the Forest Bed series, containing numerous '' pebbles " 

 of clay derived from the greatly denuded " Lower Freshwater 

 Bed," and it is from some of these " clay pebbles " that the vole 

 remains in question were collected. With the exception of two 

 fragmentary teeth {m^) of M. pliocwnicus, which ajapear to have 

 been derived from a still older deposit than that represented by 

 the " clay pebbles," all the specimens in this small series are 

 referable to M. newtoni. 



Three of the teeth from East Runton are represented in 

 Figs 99. and 103. Li a young right m^ (Fig. 103), with the pulp- 

 cavities still open below, the outer re-entrant folds are all 

 shallow transversely, so that the dentinal spaces are confluent 

 with each other ; but these folds, particularly the third one, 

 become deeper in lower levels of the crown, so that in adult 

 stages of wear the triangles would be substantially (or rather 

 tightly) closed as in the adult tooth here figured (Fig. 99, 19). In 



' Since this was written Mr. J. B. Johnson has collected some vole 

 remains from the Norwich Crag at Bramerton ; among his specimens is a 

 much worn right m^ of small size which may be referred to M. newtoni. 



^ Reid, " Geology of the country round Cromer." Mem. Geol. Surv. 

 1882, pp. 27-28 and Pliocene Deposits of Britain, 1890, p. 157. 



ff 

 I 



