FRESHWATER INVERTEBRATE DISTRIBUTION 227 



areas as the Great Basin, including Panipholyx, Carinijex, and 

 Fluminicola, but these do not constitute a large fauna. The Pele- 

 cypoda (exclusive of the Sphacriidae) are very poorly represented 

 west of the Continental Divide; for example, only about six species 

 of unionids are native to California. One of these, Margaritifera 

 margaritifera, is widely distributed in the western states but also 

 occurs in the North Atlantic states. But presumably the great 

 unionid population of the Mississippi Valley has never been success- 

 ful in overcoming the difficulties of the Divide, sandy and silty 

 rivers, and the intermittent nature of many western streams. 

 Undoubtedly a further problem in the West is the lack of proper 

 fish hosts for the glochidia stages of unionids. Isopoda and Amphi- 

 poda are apparently poorly represented in the West, although most 

 of the few species found there are endemic; the relative abundance 

 of species in the eastern states is probably a reflection of speciation 

 in springs and subterranean waters. Freshwater crayfishes were 

 originally represented by only five species of Astacinae west of the 

 Continental Divide, and this same subfamily is represented else- 

 where only in northern Europe and eastern Asia, by about ten 

 species. The Cambarinae, however, which are so abundant east of 

 the Continental Divide, have, in Recent times, apparently never 

 been successful in spreading westward from their centers of dispersal 

 and speciation in Mexico and the south-central and south-eastern 

 parts of the United States. It is, of course, entirely possible that 

 the Cambarinae inhabited the western states area in pre- Pleistocene 

 times. 



A fifth and last category includes three small groups that are 

 especially characteristic of the West and that are also abundant in 

 comparable geographic and climatic areas elsewhere in the world. 

 These are the Anostraca (fairy shrimps), Conchostraca (clam 

 shrimps), and Notostraca (tadpole shrimps). All are far better 

 represented in the West than in the eastern half of the country. 

 Two peculiarities of these groups are perhaps primarily responsible 

 for their occurrence here. First, they produce an abundance of 

 highly resistant, thick-shelled eggs that are able to withstand 

 extremely unfavorable conditions for long periods. Second, many 

 species are adapted to living in unusually saline, alkaline, and 

 silty bodies of water. Permanent and vernal ponds and pools 

 throughout the West thus afford ideal conditions for these phyllo- 



