I80PODS OF NORTH AMERICA. 409 



'. First pair of pleopoda in the male coupled with the second pair, the peduncles 

 being elongate. Second pair in the male large, the peduncles situated outside 

 of and coupled with the first pair, forming a large operculum. Third pair in 

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

 b. 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 subdorsal, not placed on peduncle-like projections 

 of the head. First pair of antennae issuing close together. Legs subequal 



in length Family XVII. JAXIRID.E 



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

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

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

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

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



more or less rapidly increasing in length Family XVIII. MUNNID^: 



b'. 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. Eyes wanting Family XIX. MUNNOPSID.S 



Family XV. ASELLID^E." . 



Body depressed. 



Lateral parts of head scarcely expanded; front without rostrum. 



Segments of thorax with lateral parts lamellarly expanded. Eyes, 

 when present, small and laterally placed. 



Both pairs of antennae with multiarticulate flagella. 



Peduncle of second antennae without scale outside of third joint. 

 First pair of legs subchelif orm ; six following pairs ambulatory; 

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

 The first b are very small, not operculif orm ; each consists of a minute 

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

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

 third pair very 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 branches single. Second pair situated below and not coupled 

 with first pair, with branches 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. 



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

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



t> Hansen writes that the first pleopoda of the female in this family are attached 

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

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

 and* of Ctecidotea, for in several species to be mentioned later I have found the 

 reverse to be true. 



