386 MICROTIN^ 



usually extensive and deep. Palate normal, nearly as in 

 Microtus. Auditory bulte moderately large, without spongy 

 tissue within, but with the postero-external part of the wall of 

 each bulla more or less thickened and strengthened by cancellous 

 bone ; stapedial artery more or less completely enclosed in a bony 

 tube in adults. 



Mandible normal, the angular processes sometimes greatly 

 reduced. Incisors more or less protruding; the lower incisor 

 passes to the labial side of the jaw between Wg and m^, displacing 

 the latter tooth lingually, and ascends the condylar process, 

 where its termination above the dental foramen is marked by 

 a more or less conspicuous hump on the outer surface of the 

 bone. 



Cheek-teeth robust, hypsodont, rootless and persistently 

 growing; their alveolar capsules protruding into the floors of 

 the orbit and braincase modifying their surroundings {e.g., pre- 

 sphenoid reduced to a slender rod of bone) and causing the mandible 

 to be unusually stout. Cement present in the infolds or valleys 

 of the teeth. Enamel usually differentiated into thick and thin 

 portions ; in earlier species the differentiation is as in Mimomys, 

 thick enamel bounding the convex sides, thinner enamel the con- 

 cave sides of the salient angles ; in later species the differentiation 

 is converse. Pattern of m^, m^, Wg ^^^ ™3 ^^^ essentially different 

 from that of normal Microtus ; m^ simplified with only three 

 salient angles on each side, the anterior loop followed by two or 

 three substantially closed triangles, the posterior loop short and 

 simple ; m^ consisting of a posterior loop, three closed triangles, 

 and an anterior loop of complex structure, with three or four 

 outer salient angles and four or five inner salient angles. 



When quite unworn the cheek-teeth have tubercular caps 

 and show various ephemeral comi)lications, which are described 

 above at pp. 107-110. 



1. t Arvicola bactonensis sp. n. 



1846. Arvicola anifhibia Owen, Brit. Foss. Mamm., p. 205 (in part). 

 1882. Arvicola am'phihius 1 Newton, Vert. Forest Bed, p. 87 (in part). 

 1885. Arvicola amphibivsLydekkeT, Cat. Foss. Mamm. B.M., pt. 1, p. 



230 (in part). 

 1890. Microtus amphibinsl Woodward and Sherborn, Cat. Brit. 



Foss. Vert., p. 365. 



Type.—B.M., No. 17634, Geol. Dept. (Green Collection). 

 An imperfect skull, both mandibular rami, and the greater part 

 of the skeleton of an adult. 



Type horizon and locality. — A lacustrine or freshwater deposit 

 at Ostend, near Bacton, Norfolk, of Late Cromerian Age. 



Range. — Known only from the type horizon and locality. 



Characters. — Size probably a little larger than in Mimomys 

 intermedins, smaller than in A. amphibius or A. terrestris; dental 



