PREFACE. 



The elaboration of the present Volume has been connected with no small 

 trouble and expenditure of time. Indeed, the group of Crustacea here treated of 

 is undoubtedly one of the most difficult, both as regards the direct examination 

 of the species, and the exact determination of the often very intricate mutual relation 

 of the several types. The group has proved to be very richly represented in the 

 Norwegian Fauna, as will appear from the extent of this Volume, (about 300 

 (291) different species, belonging to 99 genera, being treated of here); and yet 

 I have every reason to believe that many other forms will be added in the future. 

 As regards this statement, I will only point to the fact that during my last excursion, 

 in the summer of 1910, no less than about 40 additional species, several new to 

 science, were procured, and of these moreover by far the greater number were 

 derived from a single locality at Korshavn, on the south coast of Norway. 



The systematic arrangement of the -genera adopted in this work can only 

 be regarded as a quite provisional one. No doubt in the future essential improve- 

 ments will be effected also in this respect. Although the present Volume cannot 

 therefore lay any claim to being an exhaustive account of the group, I think that 

 it will at least be found useful as a basis for further investigations. 



As to the preparation of the plates accompanying this Volume, I have been 

 anxious to give, besides analytical figures of the several appendages, also good habitus- 

 figures of each species, drawn on a sufficiently large scale. The great advantage of 

 such figures for an easy determination of the species is evident. In most cases it is 

 found that earlier authors have contented themselves with giving only lateral figures 

 of the species belonging to the present group. I find, however, that in many 

 cases the dorsal view of the animal more clearly reveals the characteristic diffe- 

 rences between nearly-related species than does the lateral view. In cases 

 where only a single habitus-figure is given, I have therefore always preferred to 

 represent the animal in a dorsal aspect. In most cases, however, the animal is 



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