I8OPODS OF NORTH AMERICA. 55 



/. Mandibles with the distal half stout, very conspicuous, or with only 

 the anterior margin concealed; from the base toward the middle 

 directed forward and a little outward. 



g. Mandibles with the rather broad, more or less tridentate, cutting 

 edges meeting squarely behind the large upper lip; the secondary 

 plate and peculiar equivalent for the molar well developed. First 

 maxillae having the plate of the first joint armed with three spines, 

 that of the third with many. Second maxillae of moderate size, 

 the three free plates very setose. Maxillipeds with the palp rather 



broad, very setose Family V. CIROLANID^E 



g / . Mandibles with the distal part produced into a long prominent 

 process, the pair much overlapping; the secondary plate and 

 molar evanescent. First maxillae having the plate of the first joint 

 unarmed, of the third carrying one very long spine. Second 

 maxillae bifid, small and feeble, the free plates almost rudimen- 

 tary, with few setae. Maxillipeds with the palp narrowed, the 



antepenultimate joint elongate Family VI. EXOCOBALLANID^E 



/'. Mandibles with the distal half narrow, most or all of it concealed by 

 the upper and lower lips; from the base toward the apex gradually 

 directed inward. Mandibles usually without molar process. Apex 

 of second maxillae simple. Antepenultimate joint of maxillipeds not 



elongate'. Family VII. CORALLANID.E 



e/. Maxillipeds with the palp embracing the cone formed by the distal parts 

 of the mouth organs, the inner upper margin and apex never setose, 

 the apex and sometimes the inner upper margin, at least in the males 

 and females without eggs, being furnished with outward curved hooks. 

 /. Antennae of both pairs with well-defined peduncle and flagellum. Man- 

 dibles with the secondary plate very often visible; palp with no inflated 

 joint. Maxillipeds with the palp commonly composed of five articles, 

 sometimes composed of two articles, the last article in the latter case 



rather short, obtuse Family VIII. JEGIOJE 



f. Antennae much reduced, without clear distinction between peduncle 

 and flagellum. Mandibles with no secondary plate; palp in adults 

 with first joint or both first and second joints inflated. Maxillipeds 

 always with palp composed of two joints, last joint rather long and 



narrow, subacute Family IX. CYMOTHOID.E 



d f . Uropoda with one of the branches almost obsolete or rudimentary not 



lamelliform Family X. LIMNORIID^E 



c'. Abdomen composed of less than six segments. 

 d. Abdomen composed of two segments. Uropoda with one branch fixed, 



immovable Family XI. SPH^ROMID* 



d'. Abdomen composed of four segments. Uropoda with both branches mov- 

 able Family XII. SEROLID^: 



Family III. GNATHIIDJE." 



Thorax with only five pairs of normal walking legs in the adult. 

 Last pair of legs wanting. First pair of legs modified; in male they 

 are valve-like, arching over the ventral side of the head; in the female 

 they are smaller and more distinctly segmented; in the larval form 



"See Sars's Crustacea of Norway, II, 1899, p. 50, and Harger, Report U. S. Comm. 

 of Fish and Fisheries, 1880, Pt. 6, pp. 408-410, for characters given below. 



