302 DR. H. J. HANSEN ON THE [Kov. 29. 



4. On the Morphology and Classification of the Asellota- 

 Group of Crustaceans, with Descriptions of the Grenus 

 Stenetrium Hasw. and its Species. By H. J. Hansen, 

 Ph.D., F.M.L.S. 



[Received October 17, 1904.] 



(Plates XIX.-XXI *.) 



I. Introductory Remarks. 



The ti'ibe or suborder Asellota is in some respects one of the 

 most varied and, as to the number of species, probably by far the 

 richest of all groups of pre-eminently marine Isopoda. A perusal 

 of the portion in question of Beddard's account of the 'Chal- 

 lenger ' Isopoda (Zoology, vol. xvii.), and of G. O. Sars's ' An 

 Account of the Crustacea of Norway,' vol. ii. Isopoda, 1896-99, 

 conveys a fair idea as to the striking differences in general aspect 

 and in some structural features between the numerous genera, 

 among which we find such types as Asellus Geoff., lanira Leach, 

 M%m%na Kr., Dendrotion G. O. S., Macrostylis G. 0. S., Ischnosoma 

 G. O. S., Desmosoma G. 0. S., Munno2)sis M. Sars, Eurycope 

 G. O. S. In the work named, Sars describes 42 species referred 

 to 21 genera; Beddard has established 32 species referred to 

 15 genera, and 8 of these genera are not found in Norway. In 

 order to furnish an instance showing hoAv much remains to be 

 known, I may perhaps state here that from the seas around 

 Greenlaird, Iceland, and the Faroe Islands, the Copenhagen Museum 

 possesses more than 90 species, of which at least 60 are new to 

 science ; the major part of these new forms were secured by the 

 ' Ingolf ' in depths between 300 and 1870 fathoms. 



Sars refers his 21 genera to five families, but it will be shown 

 below that four of these are almost artificial, as really good 

 characters for their separation are wanting. But this distmguished 

 author has produced a vast number of good figures — with useful 

 descriptions — of all his forms and of details of their dermo-skeleton ; 

 moreover almost all really important genera hitherto established 

 of Asellota have been incorporated in his fine work. For these 

 reasons I can often, in the following discussion, refer the reader 

 to his figures as proofs and illustrations for my remarks. 



Among the genera not represented in the Norwegian fauna, 

 Stenetrmm Haswell is the most aberrant and important. Of 

 this genus five species have been established by four authors, 

 but our knowledge of several essential points of its structure, 

 especially of the pleopoda, is still imperfect. Most of the figures 

 of S. antillense, sp. n., were drawn by me more than fifteen years 

 ago, but publication was, however, postponed, which proved to be 

 fortunate, as new and interesting species have been received in 



* For explanation of the Plates, see p. 330. 



