316 T. D. A. COCKKKELL. 



two regions are not in the same secondary division, but from the 

 proximity of New Jersey to the boreal (or at least transition) zone, 

 there should be a fair amount of affinity. 

 The results are as follows : 



Colo, (niid-alpine, Custer ("o.) and N. J. Genera in Common Species in Common 



Rhopalocera 24 23 



Heterocera 59 31 



Heteroptera 36 27 



Houioptera 14 4 



Ortlioptera 12 9 



145 96 



The rule here gives just the results that might be expected. The 

 greater number of butterfly species in common is in accordance with 

 the wide distribution of Rhopalocera, as before mentioned. 



It would be interesting to compare the mid-alpine and sub-alpine 

 zones of Colorado in the same manner, but this cannot be satisfac- 

 torily done until the sub-alpine records are more complete. When 

 comparing distinct faunae in close geographical proximity, it would 

 be necessary to exclude from consideration all casual occurrences of 

 species out of their proper zone. The breeding areas are really what 

 should be compared. This is sufficiently obvious; for instance it 

 would be absurd, in such a comparison, to reckon such genera as 

 Jiinonia and Erebus as inhabitants of the mid-alpine zone. 



EQUIGENERIC AREAS. 



For minor divisions, to be used in relation to particular groups, I 

 have devised what may be termed equigeneric areas. 



Equigeneric areas are areas throughout which the genera of the 

 group under consideration are identical. 



These areas are sometimes large, sometimes small. When two 

 genera overlap, the region where they both occur, however small, 

 makes a separate equigeneric area. This might be thought a disad- 

 vantage; but really, I believe it to be an advantage in the method, 

 since it is important to recognize these intermediate or overlapping 

 areas. 



Taking the slugs as convenient for illustration, the equigeneric 

 areas in North America are as follows : 



(1.) Area of Agriolimax only = boreal province (so far as slugs have been found) 



and central region. 

 (2.) Area of Agriolimax -\- Philomycus = Eastern province {Veroiiicella is found 



in Florida, and if truly indigenous there, as seems likely, constitutes 



another area). 



