ISOPODA. 13 



marked with the three characteristic plates. The terminal joint is quite small, ovoid, 

 with very fine setae on its margin and stouter ones on its surface. 



The pereiopods are as usual five pairs and differ but little from each other. The 

 first two pairs are smooth generally, although the carpus of the first has three stout 

 tubercles ventrally, and the propodus bears a row of small spines along its ventral 

 border, one large pectinated spine about two-thirds of its length, and a similar but 

 larger one distally. In all other cases only the two larger spines are present, and 

 these are not so distinctly pectinated. The three posterior pairs have the bases 

 tuberculated dorsally, and the other joints are also tuberculated but to a less extent 

 ventrally. In all cases the ischium is dilated distally and the merus has a well- 

 developed lobe projecting forwards. The limb is fairly well supplied with setse of 

 varying length and strength. The dactylus is powerful. 



Female. The adult female has an enormously swollen body, and the cephalosome 

 is much smaller than that of the male and certainly not half the length. Its anterior 

 margin has a rounded lobe in the middle line, below which some of the mouth organs 

 project as a wide but truncated rostrum. The preocular spines are smaller than in 

 the male. Two anterior segments of the thorax are distinct, the following three are 

 completely fused though sometimes the lines of segmentation can be observed. The last 

 thoracic segment, which is considerably reduced in the male, is in the same condition 

 in the female. The younger individuals are much more slender, but the fusion of 

 three segments of the mesosome is equally complete ; the two anterior ones are 

 more distinct. The cephalosome is smaller still and its anterior margin is angular 

 with a truncated projection in front, and below this the mouth organs project as a 

 conical rostrum ; the precise condition of this depends on age. The pereiopoda are 

 similar to those of the male, but more slender and without the tubercular processes. 



The drawings illustrating Gnathia polaris in the ' Southern Cross ' Report were 

 made with great care, but one feature of importance has not been brought into the 

 prominence it deserves, and that is the crustaceous character of the exoskeleton of 

 the cephalon and some two or three segments of the mesosome. This, however, is 

 a very variable feature, and during an examination of the large number of specimens 

 brought back by that Expedition it also appears that the cephalic and thoracic 

 outlines of the animal are not always as depicted in the illustration. The type figured 

 requires no modification, but in other specimens where the jaws are closed the median 

 projection is not visible ; a rounded swelling appears at the base of each mandible, 

 but I have been unable to detect the two stout spines which are so characteristic 

 of the ' Discovery ' specimens. 



The preocular spines almost invariably bear a few more or less distinct subsidiary 

 spinules on the front margin. The crustaceous character of the cephalosome and the 

 first three segments of the mesosome is constant, though often much concealed by a 

 diatomaceous deposit which sometimes covers the entire animal. The cephalosome, too, 

 is more or less completely covered with very small spines ; these also occur laterally 



