308 DR. H. J. UAXSEN ON THE [Nov. 29, 



as a subfamily of the Asellidfe, but I will leave to my friends 

 Dr. Ai-mand Yire and Mr. Adrien DoUf us the further examination 

 of the structure of this most interesting type. 



Having thus discarded Stenaselhts, and omitting Asellus 

 (Mmicaselhos, Coecidothea) and Stenetruim, we shall now consider 

 the remaining portion of Asellota, which comprises, I think, 

 about thirty genera, twenty of which are found in Norway. 

 Sars divides all Asellota into five equivalent families : Asellidfe, 

 laniridse, Munnidse, Desmosomidse, and Munnopsidpe. A perusal 

 of Sars's account and of Beddard's 'Challenger' work will show that 

 the genera which have been — or must be — referred to the four 

 latter families present great differences in the shape of the body, 

 in length and shape of the thoracic legs and their coating of 

 spines or setfe, in the degree of development of the ui-opoda and 

 similar features, but of more essential differences between these 

 four families scarcely one is to be found. The peduncle of the 

 antennae is six- join ted ; tlie exopod is sometimes rather large, 

 sometimes rudimentary or absent. In the shape of the mandibles 

 the differences between the genera decidedly allied to each other, 

 and by Sars referred to his Munnopsidse, are considerably larger 

 than those which can be pointed out between the families. The 

 other mouth-parts present no difference worth mention. The 

 thoracic legs show frequently excellent generic characters, but the 

 differences are so gradually developed that they are valueless as 

 distinguishing characters between the families. I will refer the 

 reader to the good figures given by Sars of the posterior pairs of 

 legs and their development as natatory organs in Uchmo2)leu7rc, 

 Desmosoma, Pseudaj'achna, and Mionnopsis. The two former 

 genera are referred to Desmosomatidae, the two latter to Mun- 

 nopsidse ; but the development of the legs as natatory organs is 

 gradual, and the difference between these legs in Psetidaraclma 

 (with their seventh joint long) and Munnopsis (with seventh 

 joint wanting) of the same family is conspicuously larger than 

 between Pseibdarachna and Desmosoma, which are refei-red to 

 different families. The differences in the uropoda are only of 

 generic value. — The pleopoda show great uniformity in the 

 genera and families, but must be treated more in detail. 



In the females of the four families recognised by Sars, the 

 lower side of the abdomen, a more or less broad margin excepted, 

 and the three posterior pairs of pleopoda are covered by a more 

 or less vaulted operculum which does not show any suture ; it is 

 the first pair of pleopoda. The second paii* is wanting. The third 

 pair has always both rami ; the exopod is sometimes small and 

 unjointed, sometimes larger and two-jointed, in most cases it is 

 situated along the margin of the endopod, but sometimes it 

 overlaps a smaller or larger portion of this plate ; furthermore, 

 the difference in size between endopod and exopod is always at 

 least considerably smaller than in Asellus or Stenetrium, and the 

 two exopods do not constitute together a kind of operculum as in 

 the two last-named genera. The fourth pair of pleopoda possesses, 



