﻿July, 1896.] 



ARVICOLA INTERMEDIUS NEWTON. 



75 



which a third and more promiuent one rises a little below the grinding surfaec. A 

 little more attrition would give the distal loop a trilobate outline, and a little more, 

 an acuminate one, from the loss of the lateral angles; finally the median ridge 

 disappears also. 



Tlie subgenus Isodelta is considered by Professor Cope to sbow an 

 exaggeration of the characters of Fitymys. The type and only known 

 species, Microtus speotlien^ is described as follows: 



This species is represented by the entire dentition of the left ramus mandibuli, 

 with a few fragments of the adjacent bone. As already pointed out, its characters 

 entitle it to rank as a distinct section of the genus. Thus, the triangles of the inner 

 side of the anterior inferior molar are one less than in any species of the section 

 Arvicola [ = Micj^otus']. The anterior loop presents two well-marked angular basal 

 areas, while its terminal portion is regularly rounded. * * * That this is not one 

 of the species of Pitymys, in which the basal lobe of the anterior trefoil has been cut 

 off by unusual inflexion of the enamel angle, is demonstrated by the structure of the 

 second molar, which is precisely that of typical Arvicola = Microtus'], all the tri- 

 angles from the posterior being isolated and alternating, producing the formula 

 1 t 0. The third molar has the usual formula, 1-1-1, the posterior two lobes being 

 crescentic, the anterior trapezoid. 



NOTE ON ARVICOLA INTERMEDIUS NEWTON. 



In a paper entitled ' The Yertebrata of the Forest Bed Series of 

 Norfolk and Suffolk'^ Mr. E. T. Kewton describes uamerous remains 

 of a microtine rodent with w^ell-developed tangs on the molar teeth and 

 intermediate in size between Arvicola ampMbius [= Microtus terrcs- 

 tris] and the smaller voles. This animal, which Mr. i^ewton named 

 Arvicola iutermedius, has been recently referred to the genus Fhencc- 

 comys? While the species is certainly not an Arvicola [= Microtus]^ it 

 appears to be equally far removed from Phcnacomys and probabl}^ from 

 Evotomys and Fiber also. The teeth are described as follows: 



I have now before me about 40 vole jaws from the Forest Bed '^ which, although 

 differing somewhat in size, agree precisely in the patterns of their teeth. Only 14 

 of these allow the bases of their teeth to be seen, but nine of these have more or 

 less distinct fangs; the other five have no fangs, but are most probably immature, 

 as in other particulars they agree precisely. I have likewise some hundreds of iso- 

 lated molar teeth, and a very large proportion of these are fanged. * * * The 

 great variation in the size of these fanged teeth would lead one to suspect that they 

 represent more than one species, but there are no sufiicient grounds for their separa- 

 tion. * * * The patterns of the grinding teeth are so nearly like those of A. 

 amphibius as scarcely to need description, and it is on the presence of fangs in the 

 adult that the chief distinction between the two species rests; nevertheless, there 

 are a few points deserviug of notice. In one of the largest and most perfect man- 

 dibular rami (figs. 3, 3a) the entire molar series, measured along the alveolar margin, 

 is 0.33 inch (8.5 mm.). Mr. Reeves's specimen, from the Bramertou Crag (fig. 12), is a 

 little larger. The first molar has the five inner and four outer angles alternating, 

 but the anterior two are not so prominent as is usually the case in A. amphibius, and 

 the front of the tooth is somewhat more rounded (fig. 3?;). In the Bramerton jaw 

 this is especially the case (fig. 12a). All the anterior lower teeth from the ''Forest 

 Bed " series which I have seen have the infoldings of the enamel behind the ante- 

 rior prism less deep than in those examples of A. amphibius which I have been able 



1 Memoirs of the Geological Survey, England and Wales. London, 1882. 

 ^Nehring, Naturwissenschaftliche Wochenschrift, Nr. 28, July 15, 1894. 



