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A nuich more important clitlerence between /^/(?-(Satoicws and the 

 Idotcidm is however to he found in the abdomen and particularly 

 in the last pair of appendages^the uropoda. In Plireatoicus these 

 are fairly normal in character and not \-ery dissimilar from those 

 found in AseUas and other genera, while in the Idoteid<e thev are 

 specially moditied into Hat subrectangular plates to cover the lower 

 surface of the abdomen and to protect the pleopoda. In some 

 species of the Idoteidct;, however, there is a rudimentary second 

 ramus, showing that this plate is formed from the typical ui'opod 

 consisting of a basal joint and two rami, the inner ramus being 

 rudimentary or absent, and the formation of the protecting plate 

 is probably a special modification of comparatively recent date. On 

 tlie whole the affinities of Plireatoicus to the IdoteAdia may be 

 considered to be somewhat greater than its affinities to the 

 Anthuridcf. In the same way we might compare Plireatoicus with 

 the Arctu7'id(f, a group that must be placed near the Idoteido'. 



When we come to compare Plireatoicus with the Asellidoe we 

 see at once that the form of the body is very different from that 

 usually found in the Asellidie, but on examining in detail we find 

 that there is a fairly close I'esemblance in many other respects. 

 The head, antenna? and mouth parts are all in close agreement, 

 except that the head is flattened in the AseUidm ; the resem- 

 blance in the mouth parts is on the whole very close and is 

 somewhat striking, considering the great ditierence in the general 

 appearance of the animal. I have been able to take Sars' descrip- 

 tion of the mouth parts in Asellus aquaticus as my guide in 

 describing those of Plireatoicus, and to follow that guide very 

 closely. All the legs and the uropoda are also closely similai*, 

 the resendilance in some of the legs V)eing quite as close as that in 

 the mouth parts. It is in the abdomen and the pleopoda that we 

 find the greatest difierence between the two. In the Asellidib the 

 abdomen is usually depressed and formed of a single piece, though 

 traces of other segments are sometimes present, the pleopoda lie 

 closely under the abdomen and are usually protected by a more 

 or less perfect operculum formed of the first pair of pleopoda. 

 In Plireatoicus the abdomen is not depressed but somewhat com- 

 pressed laterally, the six segments are all separate and well 

 developed, the pleopoda hang vertically downwards from the 

 segments of the abdomen and there is no operculum, the pleopoda 

 being exposed below though protected laterally by the pleura of 

 the abdominal segments. 



These differences in the abdomen are pretty considerable, but 

 from the ti'aces of separate segments found in some of the species 

 of the Asellidie it is quite clear that the ancestors of the Asellidcf. 

 possessed an abdomen of six separate segments, and that these 

 have gradually coalesced to form a single plate just as we see the 

 same process going on at the present time in the Idoteidte, where 



