98 GIDEON S. DODDS. 



though nine or more of them occur in the Transition zone of our 

 northern states and may be expected to extend into the local por- 

 tion of this zone. This leaves 17 species which apparently do 

 not go above the Upper Sonoran, to which must be added some 

 others which have their greatest abundance here rather than in 

 the Transition into which they extend. The remaining species 

 of the zone, roughly half of its population, range upward into 

 other zones, 9 of them as far as the Arctic-Alpine. Thus we 

 recognize among the species of this zone two groups : the one 

 including euthermic species which range upward across temper- 

 ature lines into the higher zones, the other composed of steno- 

 thermic forms, intolerant of great temperature differences and 

 accordingly confined pretty strictly to the zone. The euthermic 

 species, while they form at least half the fauna of the zone, are 

 less likely to receive consideration on account of zonal distribu- 

 tion because they do not form its distinctive part. To this group 

 belong nearly all the species of the family Daphnidse, many of 

 the Chydoridae and four of the five species of Cyclops, together 

 with a few from other groups. 



Conspicuous in the stenothermic component of this zone are 

 two groups: (i) four species of Diaptomus and (2) eleven 

 species of phyllopods (Anostraca, Notostraca, and Conchostraca). 

 These two groups are of interest because, not only in this region, 

 but in general they are represented by stenothermic species. 

 Diaptomus forms an important part of the entomostracan fauna 

 everywhere and includes many species, nearly all of which are 

 very narrowly stenothermic and have restricted geographic 

 ranges. The phyllopod group is of special interest here, for, 

 though represented in the fauna of all parts of the country, and 

 of the world, forms a more important part here than in most 

 regions. The species of this group are especially abundant in 

 the Sonoran portion of the Austral Zone, where they flourish in 

 the transient pools of this arid region. It will be seen by refer- 

 ence to the last column of Table I., that these species are all of 

 restricted range and confined to the states which include portions 

 of the Sonoran zone. The species of phyllopods, like those of 

 Diaptomus, are nearly all stenothermic. The phyllopods belong 

 in greater abundance to the warmer zones, while Diaptomus is 

 especially plentiful in the colder ones. 



