yS E. LE ROY MICHAEL. 



cles. The table indicates the distribution of these characters 

 among the species. 



Finally the general shape of the animal is of prime impor- 

 tance. Length, breadth, proportional length of tail, extent or 

 absence of a constriction at the tail septum, are all points to be 

 noted. In some species the body tapers gradually from head to 

 tail, in others a sudden diminution occurs at the tail septum ; 

 some are of the same width throughout the entire trunk, others 

 are much wider in the middle of the trunk tapering toward head 

 and tail. Matters of this kind, while extremely useful, are diffi- 

 cult of diagnostic description so that camera drawings present 

 the most accurate means of exhibiting these relations. 



Key to the Genera of Ch.etogxatha. 



1. Two pairs of lateral fins. Two pairs of rows of teeth. Only slight epidermal 



thickening on body Sagitta. 



2. One pair of lateral fins, partly on body and tail. One pair of rows of teeth. 



Body longer than tail. No epidermal thickening behind the head Krohiiia. 



3. One pair of lateral fins, entirely upon the tail segment. Two pairs of rows of 



teeth. Prominent thickening of epidermis extending from behind head to tail. 



Spaaella. 



Key for the Determination of the Species of Sagitta to be Used for 

 Living or Perfectly Preserved Material. 

 I. Species with collarette '2. 



1. Species without collarette 2- 



2. Shaft of seizing jaw serrated Sagitta serratodentata. 



2. Shaft of seizing jaw not serrated 3- 



3. Irregular transverse septa present on trunk Sagitta inimina. 



3. Irregular transverse septa absent on trunk 4- 



4. Posterior teeth 12 to 32 in number 5- 



4. Posterior teeth I to 12 in number 8. 



5. At least 50 per cent, of the posterior fin in front of the septum 6. 



5. Less than 50 per cent, of posterior fin in front of septum Sagitta bedoti. 



6. Tail 28 to 40 per cent, of total length Sagitta macrocephala 



6. Tail 16 to 25 per cent, of total length 7- 



7. Middle third of body of equal width Sagitta bipiautata. 



7. Body much wider at middle of the length Sagitta enflata. 



8. Length of anterior fin from 44 to 66 per cent, of total length, confluent with the 



posterior fin Sagitta whartoni. 



8. Length of anterior fin less than 40 per cent, of total length, always an interval 



between anterior and posterior fins 9 • 



9. Anterior fin on a level with ventral ganglion Sagitta furcata. 



9. Anterior fin remote from ventral ganglion lO- 



10. Posterior teeth less than 8 in number Sagitta hexaptera. 



