238 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



Krohnitta subtilis (see p. 269), and KrohniteUa boureei (Germain and 

 Joubin, 1912, p. 133). 



KEY TO GENERA. 



1. Two pairs of rows of teeth 2 



1. Teeth entirely absent, or only one pair of rows present 4 



2. Two pairs of lateral fins, the posterior pair being partly on body and partly 



on tail. Fins completely or incompletely rayed; anterior and posterior 

 pairs sometimes connected by narrow membrane Sagitta. 



2. One pair of lateral fins, or two pairs of which the posterior one is entirely 



confined to the tail-segment. Fins completely rayed 3 



3. One pah of lateral fins confined entirely to tail-segment. Collarette massive, 



extending to tail-septum and spreading out over fins. Greatest width 

 slightly anterior to tail-septum, and exceeding half that of the body. 

 Ventral transverse muscles absent. Anterior and posterior teeth both 



exceed six in number Pterosagitta. 



3. One pair of lateral fins confined entirely to tail-segment, or two pairs the 

 posterior one of which is confined to tail-segment. Collarette present but 

 not massive. Greatest width slightly behind head, less than half that of 

 body. Ventral transverse muscles present in body-segment only. Neither 

 anterior or posterior teeth exceed five in number Spadella. 



3. One pair of lateral fins partly on body and tail. Collarette absent. Ventral 



transverse muscles present in both body and tail Heterokrohnia. 



4. Lateral fin begins at or in front of ventral ganglion and extends onto tail but 



never to seminal vesicles. Ventral transverse muscles present in anterior 

 third of body. One pair of rows of teeth Eukrohnia. 



4. Lateral fin begins about half-way between ventral ganglion and tail-septum, 



and extends fully to seminal vesicles. Ventral transverse muscles absent. . 5 



5. Head small but wider than body. Less than 50 per cent of fin in front of 



tail -septum. Width of body less than 8 per cent of total length. One 



pair of rows of teeth. Seizing jaws delicate, but not filliform Krohnitta 



5. Head small, narrower than body. More than 60 per cent of fin in front of 

 tail-septum. Width of body exceeds 9 per cent of total length. Teeth 

 absent. Seizing jaws filliform Krohnittella. 



KEY TO SPECIES OF SAGITTA. 



1. Collarette absent , 2 



1. Collarette present 13 



2. Shaft of seizing jaw serrated S. serratodentata. 



2. Shaft of seizing jaw not serrated 3 



3. Anterior and posterior fins confluent 4 



3. Anterior and posterior fins separated 5 



4. Both pairs of fins entirely rayless throughout at least their anterior thirds; 



tail usually exceeds 15 per cent of total length S. lyra. 



4. Fins only rayless adjacent to body but not along outer margins; tail usually 



less than 15 per cent of total length S. gazelle. 



5. Anterior fins longer than posterior fins S. philippini. 



5. Posterior fins longer than anterior fins 6 



6. Anterior fins entirely rayless; rays of posterior fin perpendicular to body. 



8. minima. 



6. Anterior fins not entirely rayless; rays of posterior fin directed obliquely 



to body 7 



7. Anterior fins extend nearly if not quite to ventral ganglion 8 



7. Anterior fins never extend within half their length of ventral ganglion 9 



8. Both pairs of fins with rays throughout; mature ovary short and thick, not 



reaching anterior limit of posterior fins 8. setosa. 



