Tribe 3. 



VALVIFERA. 



Remarks. — The chief character by which this group is defined from all 

 other known Isopoda, consists in the peculiar structure of the uropoda, which, 

 like a pair of folding doors, arch over the lower face of the metasome, thus form- 

 ing a shelter for the delicate pleopoda, the latter being for the most part branchial 

 in character, though the anterior pairs may at times also serve as swimming limbs 

 for propelling the animal through the water. We only know of 3 families belong- 

 ing to this tribe, viz., the Idotheidce, Arcturidw and Chcetiliidee, the last of which, 

 however, is still very imperfectly known. The first 2 families are represented in 

 the fauna of Norway, and will be treated of below. 



Fam. 1. Idotheidae. 



Characters. — Body more or less depressed, with the segments of mesosome of 

 uniform appearance, coxal plates sometimes distinctly defined, sometimes confluent 

 with the segments. Metasome with some, or all of the segments consolidated to 

 form a large terminal piece, carrying the normal number of pleopoda. Superior 

 antenna? comparatively small, with the flagellum uni-articulate. Inferior antennae 

 more or less elongated, and originating outside the superior, from the lateral corners 

 of the cephalon. Oral parts on the whole normal, though the mandibles are al- 

 ways without palps,^and the joints of the maxillipeds are often reduced in number. 

 Legs rather strongly built, increasing in length posteriorly, the 3 anterior pairs 



