ANNULOSA. 551 



Division II. Exterior lamella of the tail bipartite. An- 

 tennae inserted in the same horizontal line, the internal ones 

 with two seta?, the external ones with a spine-shaped squama 

 at the first joint of the peduncle. Legs ten, (anterior 

 pair largest and didactyle). 



11. ASTACUS. 15^. POTAMOBIUS, 13. NePHROPS. 



Division III. External antennae with a large broad 

 squama, or scale, at their base. Abdomen with the second 

 joint anteriorly and posteriorly produced below. Legs ten. 



Subdivision 1. External antennae inserted in the same 

 horizontal line with the interior ones, which have two setae. 

 Tail with the external lamella composed of but one part. 



14. CrANGON. 15. PoNTOPHILUS. 



Subdivision 2. External antennae inserted below the in- 

 ternal ones ; interior ones with two setse inserted in the 

 same horizontal line. Exterior lamella of the tail bipartite. 



16. Atya. 17. Processa. 



Subdivision 3. External antennae inserted below the in- 

 ternal ones ; interior ones with two setae, one placed above 

 the other. (External lamella of the tail composed of but 

 one part). 



* Internal antennae with the superior seta excavated be- 

 low. Claios spinulose. 



18, Pa ND ALUS. 19. HippoLYTE. 20. Alpheus. 



** Internal antennae with the superior seta not excavated. 

 Claws simple. 



21. Pen.eus. 



Subdivision 4. External antennae inserted below the 

 internal ; interior ones with three setae. External lamella 

 of the tail composed of but one part. 



22. Pal.emon. 23. Athanas. 



Division IV. External antennae inserted below the in- 

 ternal ones, with a large scale at their base. Legs sixteen. 

 24. Mysis. 



