IOO GIDEON S. DODDS. 



zone is not one of importance as a limit in either direction, for 

 no such significance can be assigned to the five species from the 

 Upper Sonoran which make this their upper limit or to the three 

 which find their lower limit here. Its fauna does, however, in- 

 clude a number of species in considerable abundance which are 

 common forms in the continental Canadian and other northern 

 zones. Important among these are Limnetis gouldii, Latona seti- 

 fera, Holopedium gibberum, Eurycerus lawiellatus, Acroperus 

 harp as, and Pleuroxus procurvatus. Six species of Diaptomus 

 common in this zone, D. shoshone, D. coloradensis, D. lintoni, D. 

 nudus, D. judayl, and D. leptopus var. piscines are confined more 

 or less closely and locally to the higher parts of the Rocky Moun- 

 tains, where they range through more than one zone. 



Of special interest in the Canadian Zone of this region is the 

 great abundance and very frequent association together of Dap li- 

 ma longispina (subject to great local variation) and Diaptomus 

 leptopus var. piscina, which throughout this zone form the domi- 

 nant species, with Diaptomus coloradensis as the form of next 

 importance. D. longispina, though a euthermic species and found 

 throughout the world in many varieties, has not been found in 

 Colorado below the Transition and has by far its greatest abun- 

 dance in the Canadian. D. leptopus var. piscines seems to be 

 confined to the Transition, Canadian, and Hudsonian of the 

 Rocky Mountain region, while D. coloradensis is a local species 

 and has not been reported outside the mountain region of the 

 state for which it is named. The list of euthermic species is 

 about the same as in the Upper Sonoran. 



The Hudsonian Zone. 



This is a narrow zone, both in vertical and in geographic extent, 

 and has only slight significance. Climatic conditions are more 

 severe than in the Canadian ; more snow falls and it lies longer ; 

 the summer is shorter and the lakes are colder. In these respects 

 it approaches the Arctic-Alpine. Its lakes are of two sorts : the 

 one of the same type as those common in the Canadian, the other 

 more like those of the Arctic-Alpine lakes on the direct courses 

 of the creeks like those higher up but differing from them in that 

 they are surrounded by timber and have considerable silt on 



