390 MICROTIN^E 



T?/;?e.—B.M., No. 15984a; Geol. Dept. (Green Collection) ; a 

 detached right ?Hj. 



Type horizon and locality. — Freshwater deposit at Ostend, near 

 Bacton, Norfolk ; Late Cromerian. 



Characters. — Cheek-teeth persistently growing like those of 

 A. bactonensis; distinguished from the latter species, so far as is 

 known, only by the form of ?Hj, which resembles that of Mimomys 

 majori in having a persistent third outer fold, whereas in A. 

 bactonensis and in M. intermedius the third outer fold of m^ is, 

 in adult stages of wear, reduced by insulation. 



In the type specimen (Fig. 106, i) the third outer fold is deep, 

 narrow, and quite persistent ; the third outer salient angle bears a 

 faint trace of the " prism-fold " ; the anterior loop is short with 

 rather prominent fourth outer and much reduced fifth inner angles ; 

 the three triangles in front of the posterior loop are almost tightly 

 closed ; the enamel is rather thick, but tends to be differentiated 

 in the normal way of Mimomys. This tooth is unquestionably 

 adult ; yet no trace of a coming closure of the cement spaces can 

 be seen below. 



A left m^ (No. 17627e) from the same deposit .shows one of the 

 earliest stages of wear. In this specimen some of the peculiarities 

 of form seen in the posterior loop and prisms of young Microtine 

 molars are still present, while the dentinal spaces are all confluent 

 with each other; the third outer valley is a persistent fold, and 

 there is no trace of a " prism-fold." The outer border of the 

 anterior loop shows irregularities which mark the remnants of 

 ephemeral complications. A minute islet obliquely placed on 

 the outer edge is the last trace of a fourth outer valley. The 

 fifth inner angle is still prominent, but with further wear would 

 tend to become obsolete. 



Other examples from Bacton show many of the variations of 

 form which are found in the m^ of M. majori in the West Runton 

 deposit. Con.spicuous among these are those with a well-marked 

 " prism-fold " associated with a persistent third outer valley 

 (Fig. 106, 2). Some of these specimens further show the groove 

 which, from the West Runton evidence, we know to be the last 

 vestige of the fourth outer valley. The retention of such features 

 and the peculiar differentiation of the enamel (thick portions on 

 convex, thin on concave sides of the salient angles) show clearly 

 that this species like A . bactonensis is to be regarded as represent- 

 ing a later phase in the evolution of Mimomys. Although many 

 of the Bacton teeth are beyond doubt adult, and some of them 

 probably aged, I have not been able to find any trace of closing 

 cement spaces among them. 



Remarks. — A. bactonensis and A. greenii may, in my opinion, 

 be fairly considered to be the direct descendants of Mimomys 

 intermedius and M. majori, the two most divergent of the three 

 species of Mimomys occurring in the Upper Freshwater Bed at 

 West Runton. Of a descendant of M. savini, the West Runton 



