Q 



THE CANCROID CRABS OF AMERICA ^ 



W Carpus of third maxillipeds articulates at or near antero-internal angle 

 of the merus. Body usually round or transversely oval. Male open- 

 ings nearly always coxal. In many species the right chela is always 

 larger than the left. 

 C Legs more or less distinctly adapted for swimming. Usually a small 

 lobe on the inner angle of the endopodite in the first maxillipeds. 

 The first antennae fold slanting or transverse. 



Family PORTUNIDAE. 



C2 Legs not adapted for swimming, or if so modified, then the male 

 genital duct opens sternally or runs in a sternal groove. Inner lobe 

 on the endopodite in the first maxillipeds wanting. 

 D' Fresh-water crabs with the branchial region much developed and 

 swollen. Body often squarish, but male openings coxal. 



Family Potamonidae. 



D2. Marine crabs with the branchial region not greatly swollen. 

 E'. First antennae fold lengthwise. 



Fi Carapace subcircular. Second antennal flagella either 



long and hairy or wanting. Family ATELECYCLIDAE. 



F2. Carapace broadly oval or hexagonal. Second antennal 



flagella present, short, not hairy -Family CANCRIDAE. 



E2 First antennae fold slanting or transversely. 



FK Body usually transversely oval. Male openings rarely 

 sternal. Not sharplv separated from the following 



family "- Family XANTHIDAE. 



F2 Body usually square or squarish. Male ducts open on 

 the sternum, or, if coxal, pass along a groove in the 

 sternum. Not sharply separated from the foregoing 



f a mily Family Goneplaadae. 



B^ Carpus of third maxillipeds' does not articulate at or near the inner angle 

 of the merus. Bodv usually square or squarish. Male openings sternal 

 except in Retropluma, where the duct passes along a sternal groove to 

 the coxopodite. In no species {CymopoUidae excepted) is the right 

 chela always larger than the left. 

 C Small, usually commensal crabs, with very small eyes and orbits. 

 Body usually more or less rounded—. Family Pinnolhendae. 

 O. Free-living crabs, with eyes not specially reduced and usually a 

 square body. ,, 



D' Last pair of legs dorsally placed and weaker than the others. 

 Interantennular septum very thin. No distinct epistome. 

 Exopodites of third maxiUipeds not hidden. 

 El. Front narrow. Female openings in normal position, ihird 

 maxillipeds subpediform, not covering the mouth. 



Family Retroplumidae = Ptenoplacidae. 



E2 Front moderately broad. Female openings on the sternal 



segment corresponding to the first pair of walking legs. 



Third maxillipeds cover the mouth to a large extent and 



have very small meropodites. 



Family Cymopolndae = Pahadae. 



D\ Last pair of legs not dorsally placed nor markedly weaker than 

 the rest. Interantennular septum not very thin. 

 E». A gap of greater or less size is left between the third maxilli- 

 peds. Front very or moderately broad. 

 F' Sides of the bodv either straight or very slightly arched. 

 Shape squarish. Front broad. Rarely true land 

 grabs Family Grapstdae. 



