454 E. T. Newton — Lemmings, etc. in the Thames Valley. 



Blackinore and Alston (loc. cit. p. 465), who call attention to similar 

 variation which they have noticed in this species. The second 

 lower grinder (Fig. 1, m 2) has three inner and three outer angles, 

 but this tooth is not characteristic. There are several examples of 

 the peculiar last upper grinder of this species (Fig. 2), which has 

 four inner and three outer angles, with the hinder end of the tooth 

 curved and hook-like. 



The occurrence of this species in the Thames Valley has already 

 been noticed in Mr. Whitaker's Geology of London and Parts of the 

 Thames Valley (Mem. Geol. Survey, p. 336, 1890). 



At the present day Arvicola ratticeps is found in the more northern 

 parts of Europe and Asia. 



Fig. 2. 



Fig. 3. 



Fig. 5. 



Fig. 7. 



m. 2. 



m. 3. 



Fig. 4. 



Fig. 6. 



Fig. 8. 



m. 3. 



m 2. 



m. 1- 



EXPLANATION OF FIGURES. 

 Patterns of grinding surfaces of teeth of Arvicolid* from the Brick-Earth of 

 (Jrayiord ana Enth. 



Fig. 1. Microtus {Arvicola) ratticeps, right lower molars 1 and 2. 



2. Ditto, right upper molar 3. 



3. Mpodes torqaatus, right lower molar 1. 



4. Ditto, right lower molar 3. 



5. Ditto, right upper molar 1. 



6. Ditto, right upper molar 2. 



7. My odes lemmus, right lower molar 1. 



8. Ditto, right upper molar 1. 



All the figures five times natural size. 



My odes torqaatus, Desm. 

 Mr Spurrell possesses a single example of this species from Erith, 

 and the specimen consists of a portion of a right mandibular ramus 

 with the first and third molars complete and in situ ; but only a 

 fragment of the second tooth remains. Embedded in the same 

 piece of matrix, and in natural relation to the lower jaw is a 

 portion of the right maxilla with the first and second grinders in 

 place, but the third, hinder molar is wanting. The camera-lucida 

 drawings of the patterns of these teeth (Figs. 3, 4) will be found 

 to agree so exactly with those of My odes torquatus as to leave no 

 doubt as to their specific identity. 



