198 BULLETIN NO. VII. 



or less subdivision of the fourth treffle. The anterior lower 

 molar is very long and variously formed, the anterior prism 

 being more or less complicated r the middle molar has four or 

 five prisms, the last being transverse, the others alternating; 

 the last molar consists of three simple prisms in a series. 



The skull is never more than twice as long as broad. Ante- 

 rior palatal foramina very short. Nasals short. Coronoid 

 process extending to the hight of the condyle. 



The four American subgenera are as follows : 



SUB GENUS MYONOMES, RAFINESQUE. 



Back upper molar with two external triangles, one internal triangle, an 

 anterior loop, and a posterior crescent-like figure (this with some vari- 

 ability). Middle upper molar with two internal triangles. Front lower 

 molar with three internal and two or three external triangles. Ears covered 

 by fur, rounded ; antitragus well developed. Tail longer than the head. 

 Sole six-tuberculed ; claws of forefeet small. ?ize and fur ordinary. 



1. Arvicola riparius ORD. 



2. Arvicola borealis RICH. 



3. Arvicola xanthognathus LEACH. 



4. Arvicola richardsoni AUD. and BACH. 



5. Arvicola longicaudatas MERRIAM. 



SUBGENUS CHILOTUS, BAIRD. 



Back upper molar with oue external triangle and a posterior trefoil; 

 anterior lower molar with three inner and two or three external trian- 

 gles. Ears small, orbicular, helix incurved all round. Size small. Tail 

 longer than head. Claws short. Two imperfectly known species from 

 the Pacific coast and one from Dakota. 



6. Arvicola oregonus BACH. 



7. Arvicola pallidus MERRIAM. 



8. Ai'vicola curtains COUES. 



SUBGENUS PEDOMYS, BAIRD. 



Back upper molar with one external triangle ; middle upper molar with 

 one internal triangle ; front lower molar with two internal and one ex- 

 ternal triangle. Tail about equal to head. Fore claws short. Size med- 

 ium, fur ordinary. 



9. Arvicola austerus LE CONTE. 



10. Arvicola minor MERRIAM. 



SUBGENUS PITYMYS, McMuRT. 



Back upper molar with one external triangle and a posterior trefoil; 

 middle upper molar with one internal triangle; front lower molar with 

 two internal and one external triangle. Tail shorter than head. Fore 

 claws fossorial. Size small, fur mole-like. 



11. Arvicola pinetorum LE CONTE. 



12. Arvicola quasiater COUES. 



