17 (16) Body heavily sclerotized and 



sculptured Sclerocrangon boreas 



Figure 39. — Sclerocrangon boreas; A, carapace and eyes, 

 dorsal view of adult; B, rostrum and eyes of juvenile. 



18 (/) Abdomen extended, symmetrical, pleura sometimes, and tail fan always well de- 

 veloped Section Macrura 



19 



18 (/) Abdomen bent upon itself or flexed beneath thorax; pleura usually small or absent; 

 tail fan variable in development; third legs unlike first, never chelate 



24 



19 (18) Abdomen well armed, pleura well developed; third legs chelate like first 



Superfamilv Astacidea, Family Nephropsidae, Homarus americanus, American lobster 



Figure 40. — Homarus americanus; A, abdomen and tail fan, 

 lateral view; B, tail fan, dorsal view. 



19 (18) Abdomen more or less membranous, pleura small or absent; third legs unlike first, 

 never chelate Superfamily Thalassinidea, mud shrimps 



20 



Figure 41. — Abdomen and tail fan of 

 mud shrimp, lateral view. 



15 



