ENTOMOLOGY OF COLORADO. 315 



Pacific Province. Central Province. Eastern Province. 



Agriolimax, 2 sjip. Agriolimax, 1 sp. Agriolimax, 1 sp. 



Amalia. 1 sp. Philomycus, 5 spp. 



Arioliniax, 2 spp. 

 He.sperarion, 3 spp, 

 Proi)hysaon, 6 spp. 



Subg. Phenacarion, 2 spp. 

 Anademilus, 1 sp. 

 Hemphillia, 1 sp. 

 Binneya. 1 sp. 



Of these, Agriolimax, Ariolimax, Binneya and Philomycus range 

 into Central America. Four genera are peculiar to the Pacific 

 province, two of them extending inland as far as Idaho. Not a 

 single genus, except Agriolimax, which is cosmopolitan, extends 

 across the continent. 



The shell-bearing forms tell exactly the same story. In Helix, 

 the group Lysinoe, with nineteen species, is confined to the Pacific 

 province. On the other hand, the large and characteristic groups 

 Mesodon, Stenotrema, Triodopsis and Polygyra belong to the Eastern 

 province, with a very small representation in the West. Among 

 freshwater forms, the extraordinarily rich fauna of the Eastern 

 province finds no parallel in the West. To pursue this subject 

 further would occupy too much space. I have used the Mollusca 

 in illustration because they are little able to Tuigrate, and present a 

 more forcible instance than I could select in Entomology. But I 

 should be sorry if these incomplete statements prevented anv one 

 from considering the matter impartially from other points of view. 



A METHOD FOB DEFINING FAUNAL REGIONS. 



It appears from a consideration of what has been written on faunal 

 regions, that it would be desirable if some rules could be laid down, 

 so as to leave the matter less to the discretion of the individual 

 writer. It would require a good deal of research to determine what 

 rules could be laid down, that would work, but as regards insects, at 

 all events, I have thought it possible that the following rule might 

 answer for secondary faunal divisions: 



Any two districts shall be regarded as in the same secondary faunal 

 division if the number of species common to both exceeds the 

 nwiiber of genera in common. 



In order to test this rule, I have compared some orders of insects 

 in the mid-alpine of Custer County with the lists in Prof John B. 

 Smith's New Jersey catalogue. According to accepted views, the 



TRANS. AM. ENT. SOC. XX. NOVEMBER, 1893. 



