430 SPHEROMID. 
ISOPODA. SPH @ROMIDA. 
NORMALIA. 
Genus—N AISA. (Leach). 
(Nres#za—White. Nasra—M. Edwards.) 
Generic character. With the sixth segment of the pereion 
enlarged, and dorsally produced into a strong bidentate process. 
Pleon with the upper surface irregular and setose. Uropoda 
with the articulated ramus greatly elongated. 
Tue dorsal surface of the body is smooth, but the 
sixth segment is slightly rugose, and is armed at its hind 
margin with two sharp processes extending over the tail, 
which has the upper surface irregular, with two large 
rounded tubercles in the middle: the outer plate of 
the lateral appendages of the tail is greatly elongated, 
and extends considerably backwards, whilst the inner plate 
is fixed, and transversely directed inwards, nearly meeting 
the corresponding division of the opposite appendage 
behind the middle of the hind margin of the tail. This 
peculiar structure of the tail and its appendages prevents 
the animal from rolling itself into a ball in the same 
manner as the Spherome, with which, in other respects, 
these animals are nearly allied. The nearest approach is, 
however, made to Dynamene Montagui, in which the 
sixth segment of the body shows traces of the two pro- 
cesses of Nesa in the bipartite tubercle of its posterior 
margin. 
The upper antenne are separated at the base by an 
oblong protuberance; the basal joint is thick, oblong, 
and rugose; the second joint small, with the apical angles 
acute; the flagellum multiarticulate. 
