there is a good deal of latitudinal displacement and 

 the whole seasonally oscillates to some degree in a 

 north-south fashion. 



The essential composition of this crustacean fauna 

 has been well documented since the turn of the cen- 

 tury, but there have been refinements in species 

 definitions, limits of ranges, and some new dis- 

 coveries. This key borrows heavily from the past, 

 but a new set of illustrations and restructuring should 

 make it more usable than the older keys or their 

 abridgments. 



The key is designed for biologists, biology stu- 

 dents, and informed amateurs. The format consists 

 of a series of contrasting statements almost always 

 offering two clear alternatives, although sometimes 

 supplementary information is added. The user 

 selects that statement in a couplet that fits best. This 

 will lead to a number which heads a new set of 

 alternatives. Eventually the choices lead to a 

 specific name. Names of higher categories are in- 

 cluded. Numbers in parentheses allow a user to work 

 the key in reverse, any parenthetic number being 

 that of the preceding couplet in the sequence. 



Labelled diagrams and a glossary of technical 

 terms are provided. Almost all characters employed 

 in the dichotomies are illustrated by line drawings, 

 but these may not adequately differentiate the 

 species outside the area of the northeastern United 

 States. Wherever practicable, drawings were made 

 of specimens from the northeastern United States. 

 Scales on ail figures are in millimeters. 



An annotated list of 92 species is given separately. 

 Among this number are exceedingly scarce species 

 as well as abundant ones. Only 17 of the latter are 

 large enough to be valuable in commerce, but these 

 contribute to some of our richest fisheries. The 

 shrimps, Penaeus setiferus and P. aztecus aztecus, 

 whose centers of distribution are in the Carolinian 

 Province, are commercially exploited by trawling 

 only south of Cape Hatteras. Their northern coun- 

 terparts, Pandalus horcalis and to a lesser extent P. 

 propinquus and P. moiUagui. occur in a smaller 

 trawl fishery from Cape Cod Bay to the Maritime 

 Provinces of Canada. The American lobster, 

 Homanis ameikanus, long a favorite seafood, is 

 caught mainly by trapping in cold temperate waters 

 of the continental shelf northward from Cape Hat- 

 teras. The American blue cvdh. Callinevics scipidiis. 

 which thrives in estuaries, is caught by a variety of 

 means from Long Island to Texas and marketed in 

 both hard- and soft-shelled states. 



The systematic and distributional section is fol- 



lowed by a selected bibliography of systematic and 

 general biological works at the end. 



GLOSSARY 



abdomen jointed hind part of body ; tail ; consisting 

 ideally of 6 segments; muscular, extended or bent 

 in shrimps, lobsters, etc.; asymmetrical in hermit 

 crabs; reduced and more or less permanently held 

 beneath thorax in crabs. 



udrostrcd groove groove beside base of rostrum 

 extending posteriorly along dorsal side of 

 carapace (on shrimps). 



antenna one of second pair of appendages; mov- 

 able segmented organ of sensation behind, below, 

 or lateral to antennule; the large feeler. 



antennal scale elongate, flattened, bladelike or fin- 

 like exopod of antenna; usually with distolateral 

 spine. 



antennidar peduncle proximal 3 articles of an- 

 tennule. 



antennule one of first pair of appendages; mov- 

 able segmented organ of sensation preceding an- 

 tenna; the small feeler. 



anterolateral teeth teeth on margin of carapace be- 

 tween orbit and lateralmost point (on crabs). 



article general term for segment of appendage. 



carapace dorsal and lateral shieldlike covering on 

 cephalothorax. 



carina keellike ridge or prominence. 



carpus third article from distal end of a leg. 



cephalothorax coalesced head and thorax. 



chela arrangement of distal 2 articles of a crusta- 

 cean limb in which terminal element is opposed to 

 element preceding it in such a manner that append- 

 age is adapted for grasping. 



In a true chela, elements are shaped as fingers 

 closing against each other. 



In a subchela, terminal article (dactyl) usually 

 closes against distal surface of penultimate article 

 (propodus). 



cheliped leg bearing a chela. 



dactyl terminal or distal article of a leg or maxil- 

 liped; movable finger of a chela. 



epistome a sternal plate of varying shape lying an- 

 terior to mouth between bases of antennae. In 

 Anomura the broad epistome comes in contact 

 with the carapace on each side; in Brachyura it 

 becomes firmly united with the carapace. 

 exopod lateral ramus of a biramous appendage. 



frame of mouth plates surrounding area in which 

 niouthparts repose (in crabs); composed of fused 



