CRUSTACEA MALACOSTRACA III. I I 



two pleopods of second pair independent to their sternite. The fusion of the second pair of male 

 pleopods in two genera is a very interesting morphological feature. 



As already mentioned, the fauna from the "Ingolf" area comprises 105 species, 61 of which are 

 new, and it was deemed necessary to establish 10 new genera. The sub-order is rich, and the general 

 aspect of many of the genera extremely different. The necessity of dividing it into families has long 

 been felt, and some attempts have been made. In his account, G. O. Sars divided the Norwegian forms 

 of the sub-order (or tribe) into five families, while Vanhoffen in his work on the Isopoda (and Tanai- 

 dacea) from the German South-Polar Expedition has eight families. In the above-named paper (1905) 

 I critisized the classification of Sars, showing that four of his families are "far from distinct from each 

 other"; based on a number of characters found in the pleopods I divided the sub-order into three 

 families, Asellidae, Stenetriidse and Parasellidse, the two first-named quite small, the third comprising 

 the great majority of forms, in reality uniting the four families of Sars. Especially is his family Desmo- 

 somatidae badly defined, because it comprises forms like Ischnomesus (Ischnosoma), Nannoniscus, 

 Macrostylis and Desmosoma, which, according to many features, are very distant from each other. And 

 such types as Schistosoma n. gen. and Pseudomesus n. gen. would be very difficult to place in his 

 system. In vain I have attempted to find leading characters in the shape of any organ. As the best 

 example may be taken the mandibles which in Ianira and allied genera have the molar process robust, 

 subcylindrical with the end cut off, and passing through a number of types as Pleurogonium, Nanno- 

 niscus, Macrostylis, Dcsvwsorna, Ilyarachna and Aspidoiwtus that process is gradually reduced, more 

 slender, gradually conical and smaller, until it is very small in Aspidonotus and disappears in Mun- 

 uopsis. But in Eurycope cornuta G. O. S., which by Sars is placed in the same family as the three 

 last-named genera, the molar process is well developed, thick, and differing from that in Ianira only 

 in being more obliquely cut off; in Mumwpsurus giganteus G. O. S., which is closely related to Eury- 

 cope and by Sars is referred to this genus, the molar process is only a quite low, broad and rounded 

 protuberance. 



Every attempt to divide the very numerous genera — of course including those not found in 

 our area — belonging to the four families of Sars into moderately well defined families will, in my 

 opinion, be impossible. But as a kind of arrangement is very desirable I attempt here to subdivide 

 the family Parasellidae H. J. H. into a somewhat larger number of smaller, but tolerably equivalent, 

 groups. In this way it is possible to arrange genera showing somewhat close relationship into a kind 

 of unit and point out its essential features, and the name "group" is much more neutral and somewhat 

 less exacting as to sharp diagnoses than the name "subfamily" or "family". 



Of the three families into which I divided the Asellota, the Asellidae with its single species 

 Asellus aquaticus L. known from Iceland and Greenland is omitted as being not marine. The Stene- 

 triidse have no representative; all the forms dealt with belong to the family Parasellidae. 



