NO. 2226. DISTRIBUTION OF ENTOMOSTRACA—DODDS. 77 



strictly confined to the zone, belongs primarily here and is the chief 

 differential form. It was found in 33 out of the 43 bodies of water in 

 the alpine zone and only in 6 of the 63 lakes of the montane zone. It 

 was first described from Yellowstone Lake by Forbes, and has since 

 been collected at Pikes Peak by Ward at 11,000 feet, and in the Tol- 

 land region, and may be taken as a typical alpine form. I have not 

 found it below 9,250 feet. The species of second importance is 

 DapTniia pulex (27 lakes), which, though present in all zones, and hav- 

 mg a general distribution throughout the world, seems to have a par- 

 ticular significance in this zone. The variety found here is a very 

 large form with long straight spine and more than the usual number 

 of anal spmes and of teeth in the pec ten. The striking condition is 

 the frequency with which these two species, Biaptomus shoslione and 

 DapJinia pulex, are associated together in this zone, so that the two 

 rather than either one may be said to characterize the fauna of the 

 zone. In 39 out of the 43 lakes assigned to this zone, one or both of 

 these species are found, and in 22 cases both of them. This association, 

 as we shall see- presently, gives place, in the montane zone, to another 

 equally stable one. Third in frequency of occurrence is Chydonis 

 spJiaericus (24 lakes), but as it is common in all zones and in all parts 

 of the world it does not seem to have any special significance in this 

 zone. Next in importance comes Diaptomus coloradensis (nine lakes), 

 which appears to belong in the lower part of this zone, whence it ex- 

 tends into the montane zone, where it has its greatest abundance. 

 This species, said by Marsh to be common in the mountain lakes of 

 Colorado, is closely related to D. tyrelli, a common mountain form in 

 the western United Stjites. Only one other species need be men- 

 tioned particularly, Branchinecia coloradensis (five pools). This 

 phyllopod characterizes the pools of this zone and in them makes 

 a third member of the pvlex-shoslione combination. This species, 

 common in the pools of the alpine region, has only once (9,575 feet) 

 appeared in my collections from the montane zone, and this was in 

 a pond where were also the two primary members of the alpine fauna. 

 The species was described from material near Grays Peak at 12,000 

 feet, has been collected near Leadville at 12,500 feet, and on the slopes 

 of Pikes Peak at 1 1 ,000 feet, and ranks as a typical example of a moun- 

 tain species with a restricted range. One record, however, necessi- 

 tates somewhat of a changed notion on this point. I have recently 

 received from Prof. Max M. Ellis a collection from St. Vrain, Colorado 

 (5,100 feet), dated May 30, 1912, in which are a considerable number 

 of specimens of this species. This record at once extends its range to 

 the plains, where it is possible that it is found in the early spring, 

 though not during the entire summer. The record does not, however, 

 take away from its significance in the alpme zone, where it is much 

 more common than at any other elevation. The remaining species of 



