ISOPODS OF NORTH AMERICA. 409 



First i)air of ploopoda in tlic male coujikul with the second pair, the peduncles 



heing elongate. Second pair in the male large, the pe(hineles situated outside 



of and coupled with the lirst i)air, forming a large operculum. Third pair in 



both sexes not forming an operculum. First pair in female large. 



. Last three pairs of legs not natatory. Eyes usually present. 



c Three posterior segments of thorax not sharply marked off from the four 



anterior ones, and not smaller. Caudal segment large, shieldlike. Eyes, 



when present, lateral or sulxlorsal, not placed on peduncle-like projections 



of the head. First pair of antenn.e issuing close together. Legs subequal 



in length Family XVIL Jaxiuid^e 



c' . Three posterior segments of thorax, as a rule, sharply marked off from four 

 anterior ones, and much smaller. Caudal segment nnjre or less vaulted 

 above, subpyriform. Eyes, when present, placed on the tips of lateral 

 peduncle-like projections of the head. First pair of antenme placed widely 

 apart. First pair of legs much shorter than others. Succeeding pairs 



more or less rapidly increasing in length Family XVIII. Munnid^ 



'. Last three pairs of legs natatory, with some of joints flattened and ciliated. 

 First pair of legs shorter than three following pairs. Second, third, and 

 fourth pairs very elongate. lOyes wanting Family XIX. Munnopsid^ 



Family XV. ASELLID^.« 



Body depre.ssed. 



Lateral parts of head scarcely expanded; front without rostrum. 



Segments of thorax with lateral parts lamellarh" expanded. Eyes, 

 when present, small and laterally placed. 



Both pairs of antennas with multiarticulate flagella. 



Peduncle of second antenniv without scale outside of third joint. 

 First pair of legs subcheliform; six following pairs ambulatory: 

 dactylus usually uni-unguiculate. Four pairs of pleopoda in female. 

 The first'' are very small, not opcrculiform; each consists of a minute 

 peduncle and a single rounded lamella. The second pair are wanting. 

 Three succeeding pairs consist each of two lamelliP. Outer lamella of 

 third pair ver}- large and incrusted, and forming with the correspond- 

 ing lamella of the opposite side a sort of operculum. 



Five pairs of pleopoda in male. First pair small with peduncles free, 

 short, and ])ranches single. Second pair situated ])elow and not coupled 

 with first pair, with })ranches attached to the distal margin of the 

 peduncle; inner branch not geniculate, its distal joint inflated and con- 

 taining a large cavity at its obtuse end; outer branch nearly as long- 

 as inner one, its distal joint movable. Third pair similar to that of 

 female. Last two pairs with both branches present. 



« See Sars for characters of family. Crust, of Norway, II, 1899, p. 95, and Hansen, 

 Proc. Zool. Soc. London, 1905, p. 315. 



^ Hansen writes that the lirst jileopoda of the female in this family are attached 

 far from each other. This is not true of some of the species of Mdiicascllus, as shown 

 in the figures to follow. It is also not characteristic of all the species of AseUas 

 and of Ca'cldotm, for in several species to be mentioned later I have found the 

 reverse to be true. 



