50 U.S. NATIONAL MUSEUM BULLETIN 292 



The width of the carapace is measured in its widest plane at right 

 angles to the midline. 



The width of the fi*ont is usually measured between its lateral 

 angles; in the fiddler crabs, Uca, it is measured between the levels 

 of the posterodistal angles of the proximal segments of the eyestalks. 



The fronto-orbital distance is measured from the outer angle of 

 one orbit to the outer angle of the other. 



The length of the rostrum is measured in a straight line from its 

 tip to the level of the posteriormost portion of the orbits. 



The length of each podomere of the pereiopods is measured along 

 the extensor margin with the appendage fully extended. 



The width of each podomere is measured at the widest part. 



Key to the Families of West Indian Freshwater 

 and Terrestrial Decapod Crustaceans 



1. Both first and second pairs of pereiopods ending in pincers (chelate) ; shrimp- 



like or crayfish-like 2 



Second pair of pereiopods not ending in pincers (simple) ; crablike .... 6 



2. First pair of pereiopods conspicuously longer and more robust than following 



pairs; first pair of pleopods vestigial (females) or forming exclusively 

 reproductive organs (males), not adapted for swimming; first segment of 

 antennular peduncle without prominent lateral spine or tooth (stylocerite) . 



Crayfishes, fresh water Astacidae (p. 117) 



First pair of pereiopods not longer and rarely more robust than following 

 pairs; at least outer branch (exopod) of first pair of pleopods adapted for 

 swimming; first segment of antennular peduncle with prominent lateral 

 spine or tooth (stylocerite) . Shrimps 3 



3. Lateral flaps (pleura) of first abdominal somite overlapping those of second; 



third pair of pereiopods ending in pincers (chelate) . Marine and estuarine. 



Penaeidae (p. 51) 



Lateral flaps (pleura) of second abdominal somite overlapping those of first; 



third pair of pereiopods not ending in pincers (simple) 4 



4. Second pair of pereiopods more slender (though longer) than first, carpus 



multiarticulate, chela much smaller than that of first pair. Brackish 



pools (Cuba) HiPPOLYTIDAE (p. 116) 



Second pair of pereiopods at least as robust as first, carpus not subdivided, 

 chela subequal to or larger than that of first pair 5 



5. Fingers of chelae of first and second pairs of pereiopods usually with terminal 



brushes of long hairs; if not, all pereiopods with exopods. Fresh water. 



Atyidae (p. 6) 



Fingers of chelae of first and second pairs of pereiopods without terminal 



brushes of long hairs; pereiopods without exopods. Freshwater and marine. 



Palaemonidae (p. 87) 



6. Fifth pair of pereiopods much smaller than first three pairs and ending in 



pincers (chelate) ; abdomen with uropods and either loosely flexed beneath 



body or soft, membranous, and twisted 7 



Fifth pair of pereiopods not much smaller than other pairs and not ending 

 in pincers (simple) ; abdomen without uropods and closely flexed against 

 sternum. True crabs 8 



