226 R. W. PENNAK 



only about seven have been reported from the western states; 

 presumably many of the turbid low-altitude lakes and streams 

 discourage the establishment of sponges, but very little work has 

 been done in mountain ponds and lakes that are sometimes fruitful 

 places for sponge collecting. Only about four species of hydras have 

 been reported from the West; more intensive collecting should 

 undoubtly reveal a fauna perhaps even richer than that of the 

 eastern states. The same situation applies to the Bryozoa; they are 

 easily transported from one place to another by virtue of their 

 hibernacula and statoblasts, yet the western species are poorly 

 known. 



A third category includes four taxa that are known to be well 

 represented in the West; they are also groups in which the lists of 

 western species will undoubtedly eventually be longer than those 

 for the eastern portion of the United States. The Cladocera, for 

 example, are represented by many cosmopolitan species, perhaps 

 largely owing to the viability and ease of transport of the ephippial 

 eggs; in addition, a good many species are restricted to certain 

 habitats in the western states, especially species of Moina, Camp- 

 tocercus, and Pleuroxus in alkali ponds and lakes. Cosmopolitan 

 species of cyclopoid and harpacticoid copepods are likewise abundant 

 in the West; the diaptomids, however, are more interesting because 

 of their proliferating speciation in western lakes and ponds (the list 

 of known species is growing markedly); furthermore, even a few 

 Asiatic and Alaskan species are now being reported from certain 

 cold-water habitats of the West. Although Ostracoda have been 

 subjected to relatively little collecting in the West, each new study 

 turns up numerous cosmopolitan and endemic species; further 

 systematic collecting should reveal a remarkably large fauna. The 

 same is true for the Hydracarina, especially in view of the experience 

 of European specialists; the little collecting already done in the 

 western states is ample evidence that it is an area rich not only in 

 cosmopolitan but also in endemic species, as one might assume in 

 view of the wide variety of isolated aquatic habitats. 



A fourth category includes five large taxa that are poorly rep- 

 resented in the West. It is, for example, remarkable that this region 

 contains relatively few gastropods, especially as compared with the 

 very rich fauna of the Mississippi drainage area. To be sure we have 

 an interesting assemblage of isolated genera and species in such 



