THE GKAPSOID CKABS OF AMERICA. 61 



Family PINNOTHERIDAE Dana. 



Pinnotheridea DE HAAN, Fauna Japon., Crust., 1833, p. 5; 1835, p. 34 (part). 

 Pinnotlieridae DANA, Araer. Journ. Sci., ser. 2, vol. 2, 1851, p. 289; U. S. 



Expl. Exped., vol. 13, Crust., pt. 1, 1S52, p. 378 (part). 

 Pinnothcrinae MILNE EDWARDS, Ann. Scl. Nat., Zool., ser. 3, vol. 18, 1852, p. 



139 [103] ; vol. 20, 1853, p. 21G [182]. 



Carapace often more or less membranaceous, antero-lateral mar- 

 gins entire or very slightly dentate. Front, orbits and eyestalks very 

 small, corneae sometimes obsolescent. Buccal cavity usually very 

 wide, often semicircular in outline. The merus of external maxilli- 

 peds, though often very large, is never quadrilateral, and never 

 carries the palp distinctly at the antero-internal angle ; ischium usu- 

 ally small, sometimes absent or indistinguishably fused with the 

 merus, in which case the merus lies with its long axis directed 

 obliquely or almost transversely inwards; exognath small and more 

 or less concealed. The interantennular septum, when distinguish- 

 able, is a thin plate. Male abdomen very narrow. Male openings 

 sternal. 



Small crabs, often living as commensals in bivalve mollusks, 

 ascidians and worm tubes or on the outer surface of echinoids, and 

 occasionally in gasteropod mollusks. Some are true parasites within 

 the tests of echinoids, many are free-swimming, especially males. 



The American genera belong to the two largest subfamilies, the 

 Pinnotherinae and the Pinnothereliinae (p. 127). 



Subfamily PINNOTHERINAE Milne Edwards. 



PinnotJicrinae MILNE EDWABDS, Ann. Sci. Nat., Zool., ser. 3, vol. 18, 1852, 

 p. 139 [103], restricted. 



x 



Ischium of external maxillipeds either rudimentary or indistin- 

 guishably fused with the merus to form a single piece which is 

 usually oblique, sometimes almost transverse. Palpus not so large 

 as merus-ischium. Carapace usually not markedly transverse. 



KEY TO THE AMERICAN GENERA OF THE SUBFAMILY PINNOTHEHINAE. 



A 1 . Dactyls of first 3 ambulatory legs simple, not bifurcate. Palate with 



longitudinal ridges. 

 B 1 . Carapace suborbicular or subquadrate, not strikingly wider than long. 



Legs not successively diminishing in length from first to fourth. 

 C 1 . .Carapace without two longitudinal, impressed lines leading back from 

 middle of upper margin of orbit. Female larger than male and 



usually ill-calcified Pinnotheres, p. 02. 



C*. Carapace with 2 longitudinal, impressed lines leading back from middle 

 of upper margin of orbit. Orbits ventral. Margin of front flush with 

 antennular surface and not forming a projecting hood. Male un- 

 known Fabia, p. 101. 



B*. Carapace much broader than long, anterior margin nearly straight Legs 

 diminishing in length from first to last, last very small. 



Parapinnixa, p. 107. 



