THE SPIDER CRABS OF AMERICA 331 



Genus LISSA Leach 



Lissa Leach, Zool. Misc., vol. 2, 1815, p. 69; type, L. chiragra (Fabricius). 

 lAssula Rafinesque, Amer. Monthly Mag., vol. 3, Aug. 1818, p. 272; 

 substituted for Lissa Leach. 



Carapace triangular, very convex; surface very uneven; meso- 

 gastric region especially elevated ; from it a more or less interrupted 

 ridge extends to the postero-lateral angle; sides of gastric and 

 branchial regions steep, Preocular tooth or spine present. Rostrum 

 suboblong, flattened, inclined downward, sometimes deeply divided 

 by a closed median fissure, extremity truncate, its outer ends forming 

 a small lateral lobe or tooth. Orbit with a superior and an inferior 

 closed fissure; eyes when retracted fitting into an inconspicuous cup- 

 shaped postorbital lobe. Basal article of antennae much enlarged, 

 its distal margin united with supraocular eave. Inner half or three- 

 fifths of ischium of outer maxillipeds strongly advanced; merus sub- 

 triangular, dilated outwardly; a very shallow sinus at articulation 

 of palpus. Chelipeds stout, palm compressed, enlarged, sometimes 

 carinate; fingers gaping at base in the male. Ambulatory legs of 

 moderate length, decreasing rapidly in size from the first to the last, 

 either cristate or nodose: dactyli unarmed. 



Inhabits both coasts of middle America, from Lower California to 

 the Galapagos Islands, and from the Bahamas to the West Indies; 

 also the Mediterranean. 



KEY TO THE AMERICAN SPECIES OF THE GENUS USSA 



A'. Branchial ridges not deeply divided, but rather sharp and tuberculate or 

 granulate. 

 B'. Postero-lateral margin simply concave. Outer margins of hepatic regions 



diverging posteriorly aurivilliusi, p. 333. 



B2. Postero-lateral margin sinuous. Outer margins of hepatic regions 



parallel bicarinata, p. 332. 



A^. Branchial ridges divided into two rounded lobes. 



B^ Carapace granulate on the protuberances, nearly smooth elsewhere. 

 At anterior angles of front a tooth less advanced than the submedian 



lobes tuberosa, p. 333. 



B^. Carapace everywhere granulate. At anterior angles of front, a small 

 curved spine, more advanced than the submedian lobes. 



brasiliensis, p. 335. 



ANALOGOUS SPECIES OF LISSA ON OPPOSITE SIDES OF THE CONTINENT 



Atlantic Pacific 



bicarinata. 

 brasiliensis. 



aurivilliusi. 

 tuberosa. 



