358 ISOPODA LIBERATICA, II. 



posterior pairs backwards. In none of the groups are 

 the fore legs terminated by a didactyle claw. The prin- 

 cipal character of the tribe consists, however, in the 

 structure of the tail and its appendages. This part of 

 the body consists of one large caudal shield, destitute of 

 lateral appendages, of which one or more of the anterior 

 segments exhibit evidences of being more or less im- 

 perfectly soldered together, whilst the branchial plates 

 are arranged in five pairs, entirely closed over by the 

 outer, that is, the first or anterior pair of pleopoda, which 

 shut together like the two divisions of a cupboard door, 

 and which plates have a transverse division near the ex- 

 tremity. 



The British genera included in this tribe constitute 

 two families, Arcturidae and Idotei&B, well distinguished 

 by the structure of their legs, which has induced Mr. 

 Dana to remove Arcturus from this division of the order, 

 and arrange it among his Anisopoda, a mode of distribu- 

 tion which appears to us unnatural. 



Mr. Dana has, moreover, established an additional 

 family in the sub-tribe, under the name of CJuetilida, for 

 the reception of a Patagonian species, Chcetilia ovata, 

 nine lines long, remarkable for having the sixth pair of 

 legs twice as long as the entire animal, terminating in a 

 very long bristle-like extremity, which consists of numer- 

 ous joints : the seventh pair has also a multiarticulate 

 extremity, which is, however, quite short. 



