6 ALLAN HANCOCK ATLANTIC EXPEDITION REPORT 



Key TO THE Western Atlantic Genera 

 (After Chace, 1942) 



1. Form elongate, "hippa"-lik;e; telson much longer than broad; 



movable portion of antenna removed from orbit by pro- 

 longation of basal segment, but frequently obscurely so 



Euceramus 



Form less elongate; telson usually broader than long, never 



much longer than broad 2 



2. Lateral wall of carapace broken up into two or more pieces, 



separated by membranous interspaces; front triangular or 

 transverse in dorsal view, never with projecting teeth; 

 carapace more or less subquadrate; chelipeds very robust 



and thick Pachycheles 



Lateral wall of carapace nearly always entire; if not, the front 



distinctly tridentate in dorsal view 3 



3. Basal antennal segment small, not joining margin of carapace, 



so that movable segments have free access to orbit . . 4 

 Basal antennal segment strongly produced forward and broadly 

 in contact with margin of carapace, so that movable por- 

 tion is far removed from orbit 5 



4. Front not prominent in dorsal view, either feebly convex or 



faintly trilobate; carpus of chelipeds thick and usually 

 heavily sculptured ; hand thick . . . Pisosoma 

 Front rather prominent in dorsal view, usually roughly tri- 

 angular; chelipeds depressed and usually not heavily 

 sculptured Petrolisthes 



5. Dactyls of ambulatory legs ending in a simple spine, usually 



with small, movable accessory spinules on lower margin 6 

 Dactyls of ambulatory legs armed with from two to four 

 strong, fixed spines; carapace distinctly broader than long 

 Polyonyx 



6. Front strongly tridentate in dorsal view; carapace usually 



longer than broad; chelipeds not robust or grossly sculp- 

 tured 7 



Front trilobate or simply triangular in dorsal view; carapace 

 usually broader than long; chelipeds thick and grossly 

 sculptured 8 



7. Carapace elongate, about one-fourth longer than broad; a 



strong spine on lateral margin . . . Minyocerus 



