80 E. LE ROY MICHAEL. 



8. Anterior teeth more than 4 in number Sagiita furcata. 



9. Body firm and opaque ; tail slender Sagitta bipu7ictata. 



9. Body transparent in formalin lO. 



10. Body flabby and easily compressed Sagitta furcata. 



10. Body firm il. 



11. Neck very marked Sagitta enjiata. 



11. Neck almost missing Sagitta bedoti. 



12. Irregular transverse septa present on trunk Sagitta minima. 



12. Irregular transverse septa absent 13* 



13. Neck well marked I4- 



13. Without neck 18. 



14. Jaws more than 8 in number 15. 



14. Jaws less than 8 in number 16. 



15. Collarette extends to anterior fin Sagitta zetesios. 



15. Collarette never extends over half the distance to the anterior fins. 



Sagitta planctonis. 



16. Body thickest in the middle third Sagitta siboga. 



16. Body of uniform thickness from ventral ganglion to tail 17. 



17. Collarette extends to anterior fins Sagitta fer ox. 



17. Collarette never extends over half the distance to the anterior fins. 



Sagitta robust a. 



18. Posterior teeth less than 7 in number Sagitta regularis. 



18. Posterior teeth more than 7 in number 19. 



19. Body opaque 20. 



19. Body transparent 21. 



20. Body equally thick from just behind neck to the tail septum Sagitta robusta. 



20. Body thicker in the middle third, gradually tapering toward head and tail. 



Sagitta neglecta. 



21. Body width about lo per cent, of total length, tail less than 27 per cent, of total 



length Sagitta pule lira. 



21. Body width about 5 percent, of total length, tail more than 27 per cent, of total 

 length Sagitta decipiens. 



Key for the Determination of the Species of Krohnia. 



I. Lateral fin extends from the region of the ventral ganglion to somewhat behind the 

 tail septum ; never reaches to seminal vesicles ; more than 60 per cent, in 

 front of tail septum Krohtiia hatnata. 



1. Cephalic limit of lateral fin never more than half the distance from the tail sep- 



tum to the ventral ganglion ; extends caudally to the seminal vesicles ; more 

 than 40 per cant, of fin behind septum 2. 



2. Body width about 4 per cent, of total length. Tail 30 to 40 per cent, of total 



length. Seizing jaws evenly curved, 6 to 9 in number. Teeth 9 to 13. 



Krohnia subtilis. 



2. Body width about 5 per cent, of total length. Tail 25 to 41 per cent, of total 



length (usually not over 33 per cent.). Teeth II to 15. Seizing jaws 6 to 9, 



with the convex edge made up of two curves, the junction of which makes an 



obtuse angle at a point about one fourth the length of the jaw proximad of the tip. 



Krohnia pacijica. 



