388 MICROTIN^ 



therefore that the West Runton species M. intermedius had, by 

 the date of the deposition of the Bacton beds, taken an important 

 step forwards and had developed into a species of Arvicola for 

 which the name A. bactonensis may be used. 



Skeleton. — In the type the cervical vertebra? are missing and 

 those of the thoracic and sacral regions are represented merely by 

 fragments. Eleven caudal vertebrse are preserved in sequence ; 

 they are much lighter than in recent Arvicola and form a series 

 48 mm. long. 



The fore-limbs are represented by the distal end of the right 

 scapula, with which is still articulated the entire right humerus; 

 and by the greater part of the left humerus, together with some 

 carpal and metacarpal bones. Of the hind limbs both innominate 

 bones (each damaged behind), both femora (of which the right 

 is perfect), both tibise, the right calcaneum, the left astragalus, 

 and some other bones of the feet are preserved. The proximal 

 epiphyseal sutures of the humerus and tibia, and the distal 

 suture of the femur are still visible, although the bones are other- 

 wise full grown ; in the existing ^. amphibius, however, the sutures 

 in question appear never to close fully, and both A. bactonensis 

 and the still older Mimomys intermedius probably resembled the 

 living Water Vole in this respect (cf. Pis. Ill and IV). 



The limb bones of A. bactonensis are much smaller and lighter 

 than those of A. amphibius, although they are a little larger and 

 stouter than those of M. intermedius. The humerus differs from 

 that of A. amphibius in its weaker deltoid crest, relatively smaller 

 external tuberosity, so that the smaller head lies more directly 

 upon the top of the bone, less developed supinator ridge and 

 narrower trochlea ; in all these features it agrees with the humerus 

 of M. intermedius. The ulna appears to be relatively a little 

 stouter and straighter than in A. amphibius. The os innorninatum 

 has, anteriorly at all events, much the same form as in the Water 

 Vole ; in the ilium the linea iliaca and crista glutea have similar 

 courses, but are less clearly defined than in the Water Vole, the 

 tract between these ridges being merely flattened instead of con- 

 cave. In the femur the lesser trochanter is less developed and the 

 head relatively smaller; but in other respects the bone is quite 

 like that of A. amphibius. The tibia is also much like that of the 

 latter species ; but it is relatively much shorter, being no longer 

 than the femur, and the groove for the tendon of the tibialis 

 posticus muscle is shallower and not distinctly separated from that 

 for the flexor longus digitorum.. The fibula is fused with the tibia 

 below at about the same point as in A. amphibius; its upper 

 free portion, notwithstanding the much smaller size of the fossil 

 bones, is absolutely stouter. The calcaneum and astragalus are 

 distinguished by their small size, the latter bone in addition 

 being relatively narrower than in the Water Vole. The 

 following comparative measurements bring out some interesting 

 differences : — 



