234 Zoology of Colorado 



company with Apus. When alive, they are beautiful objects. 

 Our species belong to the genera Branchinecta, Thamnoccphalus 

 and Streptocephalus. The Thamnoccphalus platyurus of Packard, 

 found at Montclair and La Junta, is a most remarkable form. 

 The head of the male has a trunk-like process, which is profusely 

 branched in the manner of a tree. The posterior abdominal 

 segments are united with the terminal piece or telson to form a 

 fin, which is deeply notched apically, and has setose margins. 

 In the other two genera the posterior abdominal segments are not 

 thus modified, and Branchinecta is quite without a frontal process 

 in the male. There are three Colorado species of Branchinecta, 

 of which B. coloradensis of Packard occurs in the mountains, to 

 over 1 1 ,000 feet, altitude, while the other two come from the 

 plains or foothills. B. packardi of Pearse is only known from 

 Colorado, the type locality being La Junta. In a similar manner, 

 Streptocephalus has a species (S. coloradensis of Dodds) in the 

 mountains, and another (S. texanus of Packard) on the plains. 

 However, S. coloradensis is not known from higher up than 

 Eldora and Ward, and does also occur on the plains, at Fort 

 Collins. 



All the above Crustacea, excepting Decapoda, Amphipoda 

 and Isopoda, are known collectively as subclass Entomostraca. 

 Those who wish to identify our species (excepting the Ostracoda) 

 will find a useful guide in "A Key to the Entomostraca of Colo- 

 rado," by G. S. Dodds, University of Colorado Studies, XI 

 (1915), No. 4. The groups of Crustacea not discussed by Dodds 

 are briefly treated in University of Colorado Studies, IX (1912), 

 pp. 47-51. In 1917 (Proc. U. S. National Museum, vol. 54) 

 Dodds published a very interesting discussion of the altitudinal 

 distribution of Entomostraca in Colorado. He shows that 

 nineteen of our species also occur in the Swiss Alps, and 

 fifteen in Northern Sweden.* The highest altitude in Colorado 

 is reached by Branchinecta coloradensis; only this and Diaptomus 

 shoshone go above 12,000 feet. Eighteen species go above 1 1,000 

 feet. The genus Diaptomus is sharply divided ecologically into a 

 group of five species which occur high in the mountains, and six 

 which occur only below 6,000 feet; but there are two others which 



*Nine of our species also occur in the Panama Canal Zone, as may be gathered from a 

 list recently (1926) published by Dodds. The average size of the species in the Colorado 

 Rockies is considerably greater than in the Panama region. 



