KEY TO ORDER DECAPODA I I 



KEY TO THE SUBORDERS OF THE ORDER DECAPODA. 



a. Rostrum seldom reduced or absent, if well developed almost invariably 

 compressed. Body always compressed. First abdominal seg- 

 ment not much smaller than the rest. First antenna generally 

 bears a scale at base; second antennal scale generally large. 

 Legs slender (except sometimes a stout chelate limb or pair of 

 limbs which may be any one of first three) with basipodite and 

 ischiopodite never fused, only one fixed point in the carpo-pro- 

 podal articulation. Abdominal limbs 1-5 always present in fulj 

 number, well developed, and used for swimming. Natantia 



aa. Rostrum often reduced or absent, depressed if present. Body not 

 compressed, generally depressed. First abdominal segment dis- 

 tinctly smaller than the rest. No scale on first antenna ; that on 

 second antenna never large, generally small or absent. Legs 

 stout, the first usually, the others never, stouter than their fel- 

 lows ; basipodite and ischiopodite almost always fused in the 

 first pair, generally in others, two fixed points in the carpo-pro- 

 podal articulation. Abdominal limbs 1-5 often reduced or 

 absent, not used for swimming. Reptantia 



KEY TO THE TRIBES OF THE SUBORDER REPTANTIA. 



o. Carapace not fused with epistome ; last thoracic sternum free, its legs 

 reduced, often chelate, always differing from the third pair. 

 Abdomen large and extended or reduced and loosely folded 

 under carapace, generally (except in Lithodidae) with biramous 

 appendages on the sixth segment. Antennae (second) well de- 

 veloped, situated external to eye, with long flagella and often a 

 movable scale. Anomura 



aa. Carapace fused with epistome at sides and nearly always also in 

 middle ; last thoracic sternum fused with others, its legs never 

 chelate, seldom markedly different from the others. Abdomen 

 much reduced, symmetrical, closely folded beneath thorax, never 

 with biramous appendages on sixth segment (only in Dromidae 

 is the sixth pair present even in a rudimentary condition). 

 Antennae (second) situated internal to eye, seldom with long 

 flagella, never with movable scale. Brachyura 



