GENUS OF TERRESTRIA.L ISOPODA. 



G-ENERAL Remarks. 



453 



The animals descriBed above present many points of interest, some 

 of which have been already briefly mentioned. Considering the large size of 

 most of them and their striking appearance, it is rather remarkable that they 

 have not been more fully des3ribed before this, but some of them occur in 

 localities not very accessible, and, as has been mentioned under V. marina, 

 even when the locality is accessible enough it is not always possible to find 

 the specimens. 



One of the most striking features is the great sexual dimorphism exhibited 

 apparently by all the species, and the varied forms that this takes ; thus in 

 T). hucculenta the male differs from the female in the possession of the 

 extraordinary balloon-like expansions of the first segment of the perseon ; 

 in D. aucMandice by the much longer and more prominent blunt spines on 

 the dorsal surface ; in D. echinata and in JD. armata by the longer spines 

 arising from the segments of the perseon ; these are extremely long in 

 B. echinata, and the capriciousness of the differences is shown in the fact 

 that, although these two species appear to be closely similar in most respects, 

 D. armata has spines on the perseon and also on the third and fourth seg- 

 ments of the pleon, while B. echinata has them only on the perseon. In 

 J?, acinosa the sexual differences are of a similar nature, but not so well 

 marked ; in D. marina the male is unknown. 



All the species are strictly sea-shore inhabitants, probably not extending 

 much above high-water mark or beyond the reach of the spray from the sea ; 

 in accordance with this they are all branchial breathers, and show no sign of 

 the adaptation for aerial respiration exhibited by most of the other terrestrial 

 Isopoda. In this respect, as well as in many points of structure, they agree 

 with the genera Scyphox and Scyphoniscus, and it is probable that their 

 nearest affinities will be found to be with these two genera. Scyphax differs 

 from Deto in the very large and well-developed eye with its rows of 

 numerous ocelli, and Scyphoniscus in the peculiar structure of the end of the 

 outer lobe of the first maxilla ; probably in both cases, however, these are 

 characters of comparatively recent origin, and do not indicate a difference 

 sufficient to counterbalance the many points of similarity. 



The most interesting point connected with the species of Deto, however, 

 is their geographical distribution, and they add a good example to the cases 

 already known of closely allied forms being found on widely separated shores 

 in subantarctic regions. D. hucculenta, from South America, which seems to 

 be quite identical with the species described as Oniscus novce-sealandice, from 

 New Zealand, emphasises the close relationship between the fauna of New 

 Zealand and that of the southern part of South America, and is paralleled 

 by the existence of the freshwater Isopod Idotea lacvstris in New Zealand 

 and its Subantarctic Islands, and also in South America at the Straits of 

 Magellan, and by the occurrence of Trichoniscus magellanicus and species 



