ISOPODS OF NORTH AMERICA. 323 



III. IDOTHEOIDEA or VALVIFERA. 



Uropoda liitcral. v;ilv('-lik(\ vciitriilly placiHl. closinji;" over th(» live 

 pairs ot" branchial ploopoda being- attaebcd on the outi'r margins to 

 the sides of the terminal segment and opening and closing like folding 

 doors. Legs of the first pair not cheliforin. 



ANALYTICAL KKY TO THE FAMILIES OF IDOTHEIOIDEA. 



a. Body narrow, elongate, Honiewhat cylindrical, scarcely depressed. Four anterior 

 pairs of legs unlike the three posterior ones, not ambulatory, nor strictly pre- 

 hensile, directed forward, slender, ciliated, the first pair very small and closely 

 applied to the mouth parts; last three pairs stouter and ambulatory with termi- 

 nal joint bifid Family XIII. Arctckid.e 



a^. Body more or less broad, d(>i)res.«ed. Legs usually nearly alike and ambulatory, 



t)ut first three pairs sometimes pronouncedly snbchcliform in structure 



Family XIV. Idotheiu^e 



Family XIII. ARCTURID.E. 



Body narrow, elongate, somewhat cylindrical, scarcely depressed. 

 First antennge with the flagellam uniarticulate. Second antennae 

 strongly developed, the peduncle having the last two articles ver}^ 

 much elongated and geniculate at the articulation of the joints: the 

 Hagcllum is short. Segments of the abdomen more or less consoli- 

 dated, the last one being rather large. The four anterior pairs of legs 

 are unlike the three posterior pairs, are neither ambulatory nor sti'ictly 

 prehensile, are directed forward, slender, feeble in structure, ciliated 

 with long delicate hairs, the iirst pair being very small and applied 

 to the mouth parts. Last three pairs of legs stouter and aml)ulatory 

 with terminal joint bifid. 



Mandibles always without palps." 



ANALYTICAL KEY TO THE CiENERA OF THE FAMILY ARCTURID^. 



a. Fourth segment of thorax greatly longer than any of the others. Marsupium of 

 female consists of two plates aflSxed to this segment. .Genus Astacilla Cordiner 

 f/. Fourth segment of thorax not greatly longer than any f)f the others. Marsu- 

 pium of female composed of three pairs of plates. 

 b. Abdomen composed of three distinct segments, two short ones anterior to the 



terminal segment Genus Arcfin-ns Latreille 



b\ Al)domen composed of two distinct segments, one anterior to the terminal 

 segment Genus Ploiroprion zur Strassen 



54. Genus ASTACILLA Cordiner. 



Body slender, cylindrical in form, with the fourth segment of the 

 thorax ver}- much elongated. Epimera small but distinct on all the 

 thoracic segments with the exception of the first. Marsupium of 

 female consists of two plates affixed to this segment. Abdomen com- 

 posed of only two segments, a single short segment anterioi' to the 

 terminal one, which is conically produced. 



«See Sars for characters of family. Crust, of Norway, II, 1899, p. 86. 



