EEPOET ON THE ISOPODA. 77 



and there are a pair of short spines, one on either side of the median line, and situated 

 exactly behind the articulation of the antennules. 



The first four segments of the thorax are subequal in length ; they increase in width 

 up to the third ; the length of the anterior region of the body comprising the head and 

 the first four segments of the thorax is about equal to that of the posterior region. 

 Each of the first four thoracic segments is concave above, the anterior and posterior 

 margins being raised into a ridge ; the anterior ridge in all these segments is prolonged 

 into a number of short spines, which have a more or less regular arrangement as follows : 

 there are four principal spines of equal length, which are disposed symmetrically and at 

 equal distances from each other and from the median dorsal line ; in the fourth segment 

 of the thorax the spines are less marked owing to the fact that they are placed a little 

 way back from the anterior margin of the segment ; they also appear to be more 

 numerous than in the preceding segments ; the lateral margins of the segments as well 

 as of their epimera are also spiny. 



The three posterior segments of the thorax increase progressively in length, 1 and have 

 the characteristic form of the genus ; the first of the segments has a few spines along its 

 anterior margin, but they are wanting on the subsequent segments. 



The abdominal shield is triangular in form, the median region is much more convex 

 than the lateral regions, from which it is separated by furrows. 



The antennules (fig. 3) are very short, not more than one-fifth of the length of the body; 

 they almost touch each other at their insertion on the front margin of the head ; the basal 

 joint of the peduncle, as is usual in the Munnopsidee, is of very great length and breadth 

 compared to the small second joint, which is at the same time the last joint of the 

 peduncle. The flagellum has seven or eight joints, of which the first is extremely long. 



The antennm were in every specimen broken off at the fourth joint ; the three basal 

 joints are as usual short and stout. 



The mandibles appear to be without a palp. 



In the thoracic limbs the fifth and sixth' pairs have the antepenultimate joint 

 extremely broad and flattened ; the penultimate joint on the contrary (fig. 6) is 

 extremely narrow and not wider than in the anterior thoracic appendages, which are not 

 modified into natatory limbs ; it is, however, like the preceding joint, fringed with long- 

 plumose hairs ; the second joint of these two limbs has a long spine on the outer side. 



The last pair of thoracic appendages are not characteristically natatory limbs ; the 

 antepenultimate joint, although to some extent flattened and therefore fitted for swimming, 

 is not nearly so much expanded and flattened as in the case of the fifth and sixth pairs 

 of appendages ; this is the principal generic character of Ilyarachna, and for this reason 

 I have assigned the present species to that genus. 



1 In the smaller specimens the last segment of the thorax is distinctly smaller than the preceding, doubtless owing 

 to the difference in age of the specimens. 



