282 PROCEEDINGS OP THE ACADEMY OP [1894. 



DESCRIPTIONS OF A NEW SUBGENUS AND NEW SPECIES OF ARVICO- 

 LINE RODENTS FROM BRITISH COLUMBIA AND WASHINGTON. 



BY SAMUEL N. RHOADS. 



Tetramerodon' siibgen. nov. 

 Type Arvicola ( Tetrai?ifrodoii) tchamcrtis Rlioads, sp. nov., Victoria, B. C. 



Sahgeneiic characters. — Dentition as in the subgenus Myuomes 

 Rafinesque'^ as restricted by Dr. Coues-(Mon. N. Ainer. Rodentia, 

 1877, p. 153), but differing therefrom in the middle upper mohir 

 lacking a [)OStero-internal trianglar loop. This tooth is composed of 

 an anterior loop, a closed antero-extei'ior triangle, a closed median 

 inner triangle, and a postero-exterior triangle. Other characters 

 as in Mynomes. See, below, dentition of A. borealis, Fig. 1. 



This section of the genus Arvlcola includes a larger number of 

 species than any other, whereas typical Mynomes is restricted to very 

 few. Among those which class under Telramerodon may be men- 

 tioned A. xatdJiogiiathus, chvoiorrhinus, borealis, loiu/icaiidus, altieolas, 

 mogollonensls, mordax, nanus, macropus, pauperrinius, townsendi, 

 tetramerus (1. c. ), edax, pJueus, and operarius. 



Of Mynomes we have A. pemisylvanicus, terrcenovoi, aztecus, and 

 drummondii f 



Prof. Baird characterized the subgenus Hemiotomys (= Myonomes 

 of Coues) as having the middle " upper molar with five closed tri- 

 angles, the last two sometimes subconfiuent," taking no notice of 

 the species then known, as edax, town send i, xantJiognathas, and 

 borealis, in which, to a greater or less degree, the posterior triangle 

 shows no indication of the subdivision seen in ^4. pennsylvanicus. The 

 four-triangled species greatly outnumber those which possess five, 

 and it is fully in accord with the system that they should be either 

 separated subgenerically from Alynomes or that this subgenus be re- 

 characterized. 



1 From Teframe^-es -^ {ouv-piivtt'd, iiud Odoiis = toi)th. 



''■ Dr. Coues' reasons for changing Ratiuesqiie's original spelling to Myoiio)nes 

 are insufficient. If retained at all, it must remain Mynomes. 



