2O2 CARL L. HUBBS. 



of its free-swimming life. As these fishes were obtained in the 

 summer of 1916, those with three annuli were born in the summer 

 of 1913, etc. 



The collecting done near Piedras Blancas and at other locali- 

 ties indicated that the males average decidedly smaller than the 

 females. It is of interest to have the field observations definitely 

 confirmed by age-determinations. 







TABLE V. 



LENGTH OF MALES OF DIFFERENT AGES. 



Two Years. Four Years. 



82 mm. (i) 89 mm. (i) 



The wide difference in the size of the two sexes of Micrometrus 

 minimus as well as of Amphigonopterus aurora (Table VI.) ap- 

 pears to be of particular significance in the case of a small vivi- 

 parous fish. The female of Amphigonopterus carries from 5 to 

 30 young which attain before birth more than one fourth the 

 length of their mother, whereas the testes of the male are rela- 

 tively small for a fish, a fact determined by the conservation of 

 spermatozoa, correlated with copulation (similar size relations 

 prevail in certain other and probably in all embiotocids, and in 

 many of the viviparous poeciliids). The differential rate of 

 growth producing the relatively smaller size of the adult males is 

 entirely or almost entirely postnatal, as previously indicated ; in 

 this connection it should again be recalled that at birth, only the 

 males are mature. 



