96 



suture along' the middle, and 2 lateral segments of a more or less oval form, 

 and carrying inside 2 peculiar movable appendages (see PI. XL and succeeding). 

 A closer examination of this compound operculum will, however, soon show, that 

 the suggestion at first put forward by Dr. Hansen is quite correct. According 

 to the opinion of this distinguished carcinologist, the male operculum is more 

 properly composed of 2 pairs of successive appendages; the median piece repre- 

 senting the 1st pair of pleopoda, and accordingly answering to the female oper- 

 culum, whereas the lateral pieces represent the copulative appendages jteculiar 

 only to the male. By such an explanation, indeed, more uniformity is obtained, 

 as to the number of appendages of the metasome, which, in fact, is the very 

 same in all Asellota, viz., 4 pairs in the female, and 5 pairs in the male, the 

 additional pair constituting the copulative appendages. In airy case, however, the 

 structure of the 2 first pairs in the forms here treated of, is very anomalous, 

 and the remaining pairs are also rather different in appearance from those in 

 other Asellota, thus justifying the establishment of a distinct family. This family 

 comprises as yet 4 genera, viz., Asellas, Mancasellus, Ccecidothea and Stenetrium, 

 all, except the last-named, containing exclusively fresh-water species. Of these 

 4 genera, only the first is represented in the fauna of Norway. 



Gen. AsellUS, G. St. Hillaire, 1764. 



Generic Characters. — Body broad, depressed, with the lateral parts of the 

 segments simple, not laciniate. Cephalon not very large, rounded, truncated in 

 front, lateral faces convex. Caudal segment very broad, slightly produced at the 

 tip. Eyes, when present, very small, consisting of a limited number of visual 

 elements. Superior antenna? scarcely longer than the peduncle of the inferior 

 ones, the latter slender and elongated, without any outer appendage to the pe- 

 duncle. Mandibles with a well developed palp,, molar expansion thick, subcylindric 

 in form. Second pair of maxillae with the outer lamellse broad, laminar, and 

 edged with numerous curved and coarsely denticulated spines. Maxillipecls having 

 in female at the base a posteriorly pointing setous lappet, epignath broad, edged 

 with, bristles. 1st pair of legs with the carpus very small, propodos much larger 

 and broader in male than in female. The succeeding pairs rapidly increasing 

 in length, and having the carpal joint well developed, propodal one linear, dac- 

 tylus spinulose inside. Uropoda with the rami subequal, slender, styliform. 



Remarks. — This genus was established as early as in the year 1764 by 

 Geoffroy St. Hillaire, and. may accordingly be regarded as the type of the family. 



