SECTION I 



INTRODUCTION 



The Asellidae are the most important surface- living freshwater isopods 

 in North America. The other isopod families which occur in North Ameri- 

 ca, Bopyridae, Sphaeromatidae (formerly Sphaeromidae) , and Cirolanidae, 

 are much less important. The Bopyridae are parasitic forms, and the 

 Cirolanidae are spring or underground forms occurring in Mexico, areas 

 immediately to its north, and Virginia. The Sphaeromatidae, otherwise a 

 predominantly marine family, includes several species found in fresh to 

 brackish waters near the coast and in hot springs. Sphaeromatids and 

 cirolanids are easily recognised by their uropods which are attached 

 anterolaterally to the abdomen, and not, as in asellids, posteriorly or 

 posterolaterally. None of these additional isopod families, all of 

 whose representatives are only rarely or occasionally encountered in sur- 

 face fresh waters of North America, is considered further in this publi- 

 cation. 



The North American fauna of the Asellidae is not well-known, but Williams 

 (1970) has revised the systematics of the surface- living species of 

 AselluSj the principal genus, and the ecology of some asellids of known 

 identity has also been studied recently (e.g. Ellis, 1961, 1971; Clifford, 

 1966; Styron, 1968; Seidenberg, 1969). 



For the purposes of this report. North American asellids are considered 

 to be represented by two genera, Liraeus and Aseltus. Other generic 

 names that have been applied include Asellopsis ^ Mancaseltus , and 

 Caeoidotea. All species of these are now regarded as either species of 

 Liraeus {=Asettopsis ^ Manaasellus) or Aseltus (= Caeoidotea) . It has re- 

 cently been proposed (Henry and Magniez, 1968, 1970) that North American 

 Aseltus species be divided between the genera Conaseltus^ Aseltus (of 

 restricted definition) , and Pseudobaioalasetlus . Of these, the first 

 two represent the elevation of former 'subgenera' to generic rank, and 

 the latter a newly proposed genus. It may well be that North American 

 asellids will prove to be a group that should legitimately be regarded 

 as representing several genera, but as the proposals of Henry and Magniez 

 were published before adequate taxonomic consideration had been given 

 surface-living species of North American asellids, and before systematic 

 knowledge of surface-living and underground species has been integrated, 

 such proposals seem decidedly premature to the present author. In order 

 not to perpetuate at generic level the sort of confusion that has exis- 

 ted in part at 'subgeneric' level in North American asellids {see 

 Williams, 1970, p. 2), the most practical procedure for the moment is to 

 regard all North American species as referable to two genera only, 

 Aseltus as defined by Birstein (1951, p. 51), and Liraeus. The former 

 contains both surface-living and underground species; species of the 

 latter typically live in surface waters only. 



