MIMOMYS 363 



of the tooth immediately in front of the prism-fold. The enamel 

 islet persists as a feature of the worn surface until a very- 

 advanced stage of wear has been reached (Fig. 100, 7) ; but the 

 groove or external vestige of the third outer valley dies out at a 

 variable distance above the level of the base of the prism-fold 

 (Fig. 100, 2a -14a ).^ Sometimes a faint sulcus is impressed upon 

 the outer surface of the fang, and this, when present, appears 

 to represent a downward continuation of the external vestige 

 in question (PI. XIII, fig. 1). 



As regards enamel pattern nio and m^ (Fig. 100, 12, 19, 20) do not 

 differ essentially from the corresponding teeth of Arvicola. The 

 outer salient angles of »i^ are quite well developed. The outer 

 re-entrant folds, in both teeth, may substantially shut off the 

 outer from the inner triangles ; but one or both folds may be 

 shallow, so as to leave a more or less wide confluency between 

 the triangles of the corresponding pair or pairs (Fig. 100, 20). 



Remarks. — The early development of the molar roots, the 

 presence of three roots each in m^ and m^, the great complexity 

 of the crown in the young TOj, and the long persistence of the 

 insulated portion of the third outer fold of in^, are characters 

 which stamp M. plioccBnicus as being a very primitive member 

 uf the genus to which it belongs. In one respect, viz., in 

 the reduction of the second inner fold of m^, it is, however, 

 highly specialized; this specialization, as a step in the wrong 

 direction, prevents us from regarding the species as the ancestor 

 of the later members of the same group. In confirmation of 

 this view we may cite the fact that at East Runton M. plioccenicus 

 occurs in association with the later forms in question {M. 

 savini and M. inter medius). 



2. fMimomys reidi Hinton. 



1882. Arvicola (Evotomys) iiilermedius Newton, Vert. Forest Bed, 



p. 85, pi. xiii, fig. 8 (in part). 

 1910. Mimomys reidi Hinton, Proc. Geol. Assoc, 21, p. 491. 

 1915. Mimomys pelenyii Mehely, Ann. Mus. Nat. Hungar., 12, p. 191, 



Taf. iv, figs. 5-8. 



Type. — Mus. Practical Geology, No. C.R. 836 ; a detached left 

 f»i, collected by Clement Reid, F.R.S. 



Type horizon and locality. — Weybourne Crag, Upper Pliocene ; 

 Trimingham, Norfolk. 



Range in, time and space. — In England known only from the 

 Weybourne Crag of Trimingham. Remains of the same or a very 



• Mr. J. B. Johnson has lent me a left mandibular ramus which he 

 found in the Norwich Crag at Bramerton. This belonged to an old 

 I individual and has the incisor, m^ and m^ in place; the molar roots are 

 I visible above the alveolar margin and the enamel islet has been worn away 

 1 from nil- The specimen is of interest, as it shows upon its outer surface 

 I insertion marks for the masseter medi<ilis muscle similar to those occurring 

 ■ in M. intermedius and in Arvicola. 



