ARVICOLA 387 



measurements as in M. intermedius, but skeleton more robust. 

 Cheek-teeth exactly like those of M. intermedius in enamel pattern, 

 and in the details of the process by which the adult pattern of 

 m^ is developed from its unworn tubercular cap ; but persistently 

 growing as in the genus Arvicola, not developing roots when adult 

 or in old age as in Mimomys. 



Description of type skeleton. — The skull is represented by the 

 maxillo-palatine region with all the cheek-teeth in place; in 

 addition there are crushed fragments of other parts, but these 

 are too imperfect for study. The palatal structure, form of the 

 maxillary root of the zygoma, and position of the anterior end 

 of the jugal are, as in Mimomys intermedius, essentially as in 

 Arvicola. The maxillo-palatine suture is a little more forwardly 

 placed than in M. intermedius, being in line with the front of m^ 

 instead of opposite the middle of that tooth ; in this the fossil 

 agrees with recent Arvicola. Both rami of the mandible are 

 present and of the usual form; each unfortunately is injured 

 behind, although all the cheek-teeth are in place. Several other 

 palatal fragments and lower jaws from Bacton are available ; 

 so far as they are preserved they agree with those parts of the 

 type skeleton, but do not give any further information. The 

 frontal region of the skull is unknown, so that we cannot say 

 whether the temporal ridges met in the middle line above to 

 produce a median interorbital crest as in Arvicola, or remained 

 low down and widely separated as in Mimomys. In size the 

 skull and jaws agree exactly with those of M. intermedius. 



So far as pattern, structure, and size are concerned there is 

 nothing to distinguish the teeth of the Bacton fossil from those of 

 M. intermedius. The enamel is differentiated as in Mimomys, 

 being thicker on the convex, thinner on the concave sides of the 

 salient angles. In m^ the dentinal spaces are similarly rather 

 confluent and the second inner fold is persistent ; m^ agrees 

 exactly in form with that of M. intermedius; similar characters 

 are shown by a great many detached teeth from Bacton in 

 the Green Collection. Part of a young left m^ shows that 

 the adult pattern of this tooth is developed from the unworn 

 tubercular cap in precisely the same way as in M. intermedius, 

 for the inner part of the reduced third outer fold is represented 

 by the last trace of an ephemeral enamel " islet." In recent 

 species of Arvicola also, m^ is frequently quite similar in form 

 to the adult m^ of M. intermedius ; but in these recent species 

 the adult pattern is developed differently, the inner part of 

 the third outer fold never being reduced by insulation. The 

 only reason for separating the Bacton animal from M. inter- 

 medins that can be found in the dentition is that in no specimen, 

 although many adult teeth from the deposit are before me, 

 can the slightest sign of failure of the dentinal pulps or of closure 

 of the cement spaces be seen. The cheek-teeth thus seem to have 

 acquired the power of persistent growth and we must conclude 



