22 



THE AMERICAN NATURALIST [Vol. XLVI1 



dentata, 8. elegans and 8. macro cephala are opaque. 

 While the three opaque species are unmistakably dis- 

 tinct, we find that, in the transparent group, 8. enflata 

 and S. hexaptera form one closely related "couplet" 

 while S. lyra and 8. gazelles make another. This is shown 

 more clearly below: 



TABLE H. 



All told, then, we have in the genus Sagitta five closely 

 related "couplets" of species. It is not to be presumed 

 that every "couplet" expresses the same degree of close- 

 ness between its two members, for such is not the case. 

 Unquestionably the two species most closely related are 

 8. neglecta and 8. regularis, and the two least so — S. 

 ferox and 8. planktonis. Now if we list these "couplets" 

 in one column and the species so far taken from the San 

 Diego region in another, the interesting fact is evident 

 that, except in one case, the San Diego Sagitta contain 

 only one species of each "couplet." Such lists are given 

 below. 



San Diego Sagitta 

 S. neglecta 

 S. bipunctata 

 S. lyra 

 S. enflata 

 8. hexaptera 

 S. planktonis 

 8. serratodentata 



"Couplets" 

 S. neglecta-S. regularis 

 8. bipunctata-S. decipiens 

 S. gazella-S. lyra 

 8. enflata-S. hexaptera 



S. planhtonis-S . ferox 



