248 



THE AMERICAN NATURALIST 



[XoL. XLI 



age of coincidence 80%. Of the 65 collections 55 were made at 

 the surface. Four of the five species thus have a coincident dis- 

 tribution, including the mostly closely related couplet ferox- 

 rohusin and the very closely related S. hedoti and S. serratodentata. 

 The hedoii-sibogos couplet appear to have a contiguous distribution 

 in the upper and lower levels, respectively, in this region. 



A second group of species which show considerable resemblances^ 

 to each other are S. hexaptera, an oceanic, stenohyaline, eury- 

 bathic, and eurythermal form; S. enflata, a warm water form of 

 wide distribution in the e})ij)lankton of warm-temperate and 

 tropical seas; and S. pidchra, a neritic surface form from the 

 Malay and Maldive Arcliipelagos. 



The accompanying table indicates the relationships of the 

 three species of the hexaptera group as suggested by the quantita- 

 tive characters. 



hexaptera 15-70 20-25 6-8 8-4 2-7 



pulchra 9-22 18-27 5-7 6-9 10-15 



euflata 22-26 16-22 7-9 7-10 12-17 



An examination of the 'Siboga' lists shows that the three species 

 occur together irs 24 catches, two in 26, and but a single one in 26, 

 the {)er(rentage of (.'oincident occurrence l)eing ()(>%. The most 

 closely related couplet in this group is pulchra-enfiata, the former 

 a neritic, the latter an oceanic si)ecies. 'I'liese two occur together 

 in the Maldi\^s and also in the 'Siboga' collections, where *S. 

 enflata is one of the most abundant species. It is found in every 

 one of the 34 collections in which S. pulchra appears. Of the 34 

 coincident occurrences 29 are in surface collections. Those three 

 related species have here a coincident distribution and N. Ju.ra pfera 



A third grony) of related species includes N. hi putivfafa , and two 

 (•()nj)lets of most closely relate<l species, [areata- pldiictoii Is and 

 iHujh'cfa -re(/idari.s. I'nblislu'd records indicate that tli(> first 

 named species is a cosmoj)(>litan one of wide range. 1 )ifliculti(vs 

 attend its specific determination so that Fowler is of the opinion 

 that it is possibly only an Atlantic neritic form not occurring in 

 Indo-Pacific waters. 



