EVOLUTION OP MOLARS 113 



first closed triangle ; this little feature is seen again and again in 

 the young teeth of many members of the subfamily, and, as the 

 young «4 of Dicrostonyx (Fig. 57) shows, it is the last trace of 

 one of the median tubercles. 



In the succeeding stage (Fig. 60b) the third outer fold has ac- 

 quired a normal form and direction and the fourth and fifth triangles 

 have become broadly confluent, their common dentinal core being, 

 however, shut off from that of the anterior loop. In middle age, 

 as Figs. 60c and d show, an interesting change, without a parallel 

 in modern Arvicola, takes place on the grinding surface of the m^ 

 of M. pliocfBuicus. The third outer valley is reduced by insulation, 

 its inner portion being converted into an enamel islet which per- 

 sists as a conspicuous feature of the grinding surface for a very 

 longtime. Gradually that islet wears out ; in old age (Fig. 60e) it 

 disappears ; and thus the tooth, in its last stages of wear, acquires 

 substantially the appearance that it has through all the adult 

 stages of wear in Arvicola. After the insulation of its internal 

 portion, the outer part of the third outer valley is represented by 

 a vertical groove on the side of the crown ; this groove dies out 

 below above the level of the base of the " prism-fold " and of the 

 bases of the hinder outer cement spaces ; this feature is clearly 

 shown in Figs. 60a', b' and c' as well as in PI. XIII, fig. 1. From the 

 latter illustration it will be seen that some trace of this external 

 vestige is sometimes continued downwards beyond the limits of 

 the enamelled crown on to the surface of the dentine forming the 

 anterior fang, a circumstance which shows how such groove-like 

 vestiges of ancient valleys tend to persist long after they have 

 ceased to be of any functional importance. 



The progressive reduction of the m^ in a later species, M. 

 intermedius, is illustrated in Figs. 61a^, which are arranged 

 in order of age beginning with the youngest. It will be 

 seen from these drawings that towards its summit the crown 

 of »!i in M. intermedius agrees closely in structure and form 

 with that of the younger specimens of M. plioccBnicus, possessing 

 e.xactly the same complications. As revealed by the successive 

 stages of wear this initial complexity is got rid of in deeper levels 

 of the crown in exactly the same way in both species. But in 

 M. intermedius the complexity is wholly ephemeral instead of 

 being almost life-long, and the adult simplified pattern, corre- 

 sponding closely with that which first appears in M. plioccpnicus 

 in old age, appears on the grinding surface of the tooth long before 

 the cement spaces show the least signs of closure below. The 

 anterior costa of the " prism-fold " is, however, weak in M. 

 intermedius, and therefore, after the insulation of the third outer 

 fold, no feature can be seen on the outer surface of the crown to 

 distinguish the external vestige of the reduced valley from the 

 concavity of the " prism-fold." But in a closely allied and 

 associated form, M. savini (Fig. 99, 13-16; Fig. 101, 1-14), 

 the costa is distinct, and an adult pattern and structure precisely 



V.L. I 



