EEPOET ON THE ISOPODA. 89 



of shorter spines ; in the female there is an additional pair of spines shorter still and 

 situated outside of them. 



Of the first four thoracic segments the fourth is the longest in the male ; in the 

 female all four are subequal. Each of these segments is traversed by a posterior ridge, 

 which is narrow dorsally and only occupies the posterior extreme of the segment ; 

 laterally this ridge becomes wider and comes to occupy the whole of the surface ; it is 

 here divided by furrows into several convex areas, which are roughened and tubercular, 

 besides bearing long spines. In the first three thoracic segments in the male the ridge 

 bears three pairs of long spines situated at approximately equidistant intervals ; the 

 median pair are the shortest and the rest increase progressively in length up to the 

 most laterally placed pair, which are situated just dorsal of the epimera. 



The fourth thoracic segment of the male only possess two pairs of long spines upon 

 the tergum, a median pair and lateral pair, situated just above the epimera and 

 considerably anterior to the median pair, owing to the increased length of the segment. 

 The epimera of this segment has also a long spine, which is covered and concealed on 

 a superficial view, by the lateral spine of the tergum. The epimera of the anterior 

 segments have no such a spine. 



In the female the first thoracic segment has four pairs of spines, the rest having 

 them as in the male ; there are also a number of short sharp spines developed between 

 the principal ones ; the lateral margins of the epimera are also furnished, in all these 

 segments, with several short spines, that on the last being the longest. On the fourth 

 thoracic segment, as in Arcturus glacialis (p. 87), the epimera are particularly large and 

 project inwards, nearly meeting in the median ventral line ; besides the lateral spine they 

 give off a large posterior curved spine, which is closely applied to the ovigerous lamella? 

 and doubtless contributes towards their support ; there are rudimentary equivalents of 

 this spine on the anterior segments. 



Each of the first four thoracic segments in both sexes has a short ridge in front of 

 the posterior one, from which arise a single row of tubercles, blunter in the male and 

 sharper in the female ; in the first segment this ridge is wider antero-posteriorly than in 

 any of the following segments. 



Of the three posterior thoracic segments the first is the longest in both sexes ; the 

 posterior portion of each segment is ridged, the ridge being narrow dorsally and 

 wide laterally ; it is covered with tubercles, which again are more strongly marked in the 

 female than in the male. On the sides of the tergum, anterior to the ridge in the first, 

 is a single conspicuous tubercle ; beneath this, and situated a little behind the articula- 

 tion of this segment with that in front, is a strong ridge with a sharp forwardly directed 

 edge traversing the ventral side of the segment ; uniting the tubercles of either side, at 

 the posterior margin of the segment, is a median V _sna P ec i notch, on either side of 

 which are a pair of short spines ; the two remaining segments have a single ridge in 



(ZOOL. CUALL. EXP. rART XLVIU. — 1886.) Bbb 12 



