97 



From the nearly-allied genus Mane .iselliis Harger it is prominently distinguished, 

 among other things, by the presence of well-developed mandibular palps. Five 

 species of this genus have been recorded, only one of which belongs to the 

 fauna of Norway. 



Asellus aquaticus (Lin.). 



(PL XXXIX.) 



(htiscus aquaticiis, Linne, Fauna Svecia, ed. 2da, p. 500. 



S}-n : Sqnilla aselhts, de Geer. 

 Oymothoa aquatica, Fabr. 

 Asellus vulgaris, Latr. 

 IdotJiea aquatica, Latr. 



Specific Characters. Body oblong oval, in male slightly widening behind, 

 in female with the greatest width about in the middle, and equalling half the 

 length, not including the caudal segment. Cephalon broader than it is long, with 

 a slight setotis prominence on each side near the base, frontal edge straight. 

 Segments of mesosome of nearly uniform size, lateral parts transversely truncated 

 at the tip, and clothed with stiff hairs. Caudal segment about half the length of 

 the mesosome, rounded quadrangular in form, and fringed all round with stiff 

 hairs; terminal edge bisinuate. with an obtuse median prominence. Eyes consist- 

 ing each of only 4 visual elements, ocular pigment not confluent. Superior antennae 

 about the length of the peduncle of the inferior ones, flagellum composed of 

 10 12 articulations. Inferior antennas not quite as long as the body, flagellum 

 more than twice the length of the peduncle. Legs densely clothed with spiniform 

 bristles; 1st pair rather short, with the propodos in female oblong oval, in male 

 subtriangular, inner edge forming an obtuse prominence, armed with 3 strong 

 spines. Uropoda with the rami more than twice as long as the basal part, and 

 edged with scattered spiniform bristles. Colour very dark, fuscous, spotted with 

 white. Length of adult female 8 mm., of male 12 mm. 



Remarks. This form has been treated of in detail by the present author 

 in one of his earlier works: "Crustaces d'eau douce de Norvege", to which the 

 reader may lie referred for detailed information on the more minute anatomy 

 and the development. 



Occurrence. The species would seem to occur everywhere in Norway, 

 especially in small ponds that have a muddy botfflm, and are covered at the sur- 

 face with Lemna. It is also occasionally met with in slow streams, and in larger 

 lakes, but never at any considerable depth. 



Distribution. The European continent everywhere, Siberia, Algieria (the 

 present author). 



13 Crustacea. 



