No. 553] DISTRIBUTION OF THE CH2ET0QNATHA 43 



did not appear to be fully mature. In- every other haul 

 only one species was represented by sexually mature 

 individuals. 



The following table, which contains data relative to 

 hauls made with horizontal closing nets, shows no in- 

 stance of two s]3ecies having been taken in the same haul 

 both of which were represented by sexually mature indi- 

 viduals. 



The facts revealed in Tables XI and XII, when taken 

 together with the foregoing discussion of vertical distri- 

 bution, suggest that the various species reach maturity 

 for the most part during different reasons, and that fer- 

 tilization probably takes place in different strata of 

 water according to the species. In the case of S. enjlata, 

 for instance, fertilization unquestionably takes place be- 

 tween the surface and ten fathoms and then only during 

 the winter, if at all, in the San Diego region. With S. 

 bipunctata, on the other hand, evidence is at hand [see 

 Michael ('11)], which space forbids presenting here, 

 showing that the species maintains a "center of migra- 

 tion" between 15 and 20 fathoms, from which center the 

 species moves up and down in response to variations in 

 light, temperature, salinity and other factors of its en- 

 vironment, which facts indicate that fertilization is 

 mainly, if not exclusively, confined to this depth of 15 to 

 20 fathoms. In the case of S. hexaptera only the imma- 

 ture have been taken above 50 fathoms, which shows that 

 fertilization must take place below this depth. Again, 

 only the very immature of 8. serratodcniata have been 

 taken above 100 fathoms, except at night when the larger 

 specimens ascend to 50 fathoms. Similarly with 8. lyra, 

 the larger more nearly mature specimens do not occur 

 above 200 fathoms to any extent, and so on with the other 

 species. 



It is quite true that much more knowledge is needed 

 concerning the vertical distribution of most of the species 

 before positive conclusions relative to the depth at which 

 fertilization occurs can be advanced. Were the deeper 

 water (below 350 fathoms) thoroughly investigated, dif- 



