REPORT ON THE ISOPODA. 31 



The single specimen is a female, and is considerably smaller than the single specimen 

 of the last species, measuring about 2 mm. in length. 



The head is small and is almost enclosed by the following segment of the thorax ; the 

 frontal margin is straighter than in the last species, and the articulation of the antennae is 

 not so near to the posterior boundary of the head ; the hinder margin of the semicircular 

 notch which encloses the basal joint of the antennules is prolonged further outwards, and 

 uniting with the posterior margin of the head, forms a narrow pointed process which 

 extends laterally nearly as far as to the spiny epimeron of the first segment of the 

 thorax. 



The thoracic segments are so very like the last species that no particular description 

 appears to be necessary ; the epimeral spine of the first segment, however, is much 

 smaller in proportion to the following spines than it appears to be in Pleurogonium 

 albidum. 



The most characteristic and obvious difference between the two species is in the 

 abdominal shield ; this is preceded in the present species, as in Pleurogonium albidum, 

 by a free abdominal segment, not produced laterally like the three posterior segments of 

 the thorax. The general shape of the abdominal shield in Pleurogonium serratum is 

 like that of Pleurogonium albidum, only the posterior termination is blunter ; the lateral 

 margins of the caudal shield as far back as the articulation of the uropoda are strongly 

 serrate, the serrations commence gradually, and somewhat behind the boundary line 

 between the abdominal shield and the free abdominal segments ; they terminate, however, 

 abruptly just before the articulation of the uropoda ; there is no trace of such serrations 

 in Pleurogonium albidum except along the posterior extremity of the abdominal shield. 



The lengths of the antennules and the antennae are as 5:6; these two pairs of 

 appendages are in fact very nearly equal in length ; there is not at any rate the great 

 disproportion between them that has been referred to in Pleurogonium albidum ; in this 

 respect the present species not only differs from Pleurogonium albidum, but is a more 

 typical species of the genus according to its definition by Sars. 



The structure of the antenmdes presents no differences from the last species. 



The antennas differ from those of Pleurogonium albidum, not only in their smaller 

 size, but in the fact that the flagellum is longer than the terminal joint of the peduncle 

 instead of shorter ; the two distal joints of the peduncle are also much more slender than 

 the proximal joints. I cannot, however, lay any great stress upon the specific differences 

 indicated by the antennary appendages. The appearance of these appendages was greatly 

 altered by the mounting of the specimen in glycerine jelly. 



The uropoda are biramose. 



Station 149k, off Christmas Harbour, Kerguelen, January 29, 1874 ; depth, 120 

 fathoms ; volcanic mud. 



