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, Arcturide and Idoteide, 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 Chetilide, for 
the reception of a Patagonian species, Chetilia 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. 
Le +o 
