Livoeus was revised taxonomically by Hubricht and Mackin (1949) who in- 

 cluded a key, but the revision is not entirely satisfactory in that con- 

 siderable emphasis was placed upon a number of apparently diagnostic 

 characters which in fact are subject to great variation and intergrada- 

 tion between species (Styron, 1969). The genus, according to Hubricht 

 and Mackin (1949) , lacks the sort of singular characters provided for 

 Asellus by the structure of the male sexual pleopods (see below) , and 

 species recognition is based upon the evaluation of many characters. In 

 the interests of providing some useful indications of the ecological and 

 geographical distributions of Liraeus species as described, a summary of 

 relevant data, abstracted mainly from Hubricht and Mackin (1949) , has 

 been compiled and is given following the similar summary for Asellus com- 

 piled from Williams (1970) (see Species List and Distribution below), but 

 no attempt is made to provide a key for their identification here. 



The ecological status of Asellus in North America is not clear. In most 

 of Europe, Asellus (as A. aquaticus L.) is characteristically present 

 within given sections of organically polluted rivers. Kolkwitz and Mar- 

 sson (1909) noted that it is one of the organisms abundant in the so- 

 called a -mesosaprobic polluted zone in particular but also occurs in the 

 3-mesosaprobic zone (the 'a- and 3-mesosaprobic zone' proposed by 

 Kolkwitz and Marsson (1909) may be regarded as roughly equivalent to the 

 'recovery zone' of several American water pollution biologists), and 

 Hynes (1960) noted that Asellus is a member (with tubificids and chiron- 

 omids) of the 'pollution fauna' in the badly polluted zone of rivers 

 affected by organic wastes. Many investigators of North American pollu- 

 ted rivers make no reference to the genus in published accounts of results, 

 whereas others do indicate its occurrence in organically polluted rivers; 

 Bartsch (1948) and Bartsch and Ingram (1959), for example, indicated that 

 it is characteristic of the 'zone of recovery'. In part, some of this 

 ecological uncertainty may be a reflection of the formerly unclear syste- 

 matic position of surface- living forms. 



Because this key to Asellus has been prepared for use by biologists con- 

 cerned with investigating pollution of inland waters, only surface- living 

 forms are discussed. Neither this key nor the more detailed account of 

 Williams (1970) should be regarded as definitive; undoubtedly much re- 

 mains to be discovered concerning the systematics of North American asel- 

 lids. It is felt, nevertheless, that the key does deal with those species 

 that are most likely to be encountered. 



North American species alone are considered, i.e. species occurring in the 

 United States and Canada. Asellids are known from Mexico (Cole and 

 Minckley, 1968), but are not considered here. 



