66 



J. M. ESSENBERG. 



tenth and the utmost length is reached by the eighth ray. In the 

 adult male the length of the sword approximates the total length 

 of the fish. The pigmentation of the sword is very striking. The 

 middle rays are from greenish to orange in color ; this is bounded 

 on both sides by deep black. The dorsal portion of the caudal fin 

 has a yellowish hue. 



The pelvic fins of the male differ from those of the female in 

 the relative length of the rays. The length of the first or anterior 

 ray of the pelvic fin of the male is to the second ray as 1:2, while 

 the reverse is true of the female i.e., as 2:1. These differences 

 appear late in sex-differentiation, approximately at the beginning 

 of spermatogenesis. 



VIII. SEX-RATIOS. 



The sex-ratio in the immature fishes between the sizes of 10-26 

 mm. in total length is given in Table II. These limits were chosen 

 because sex is unmistakably established at 10 mm. and normal adult 

 females appear at about 27 mm. in total length. The specimens in 

 this table were taken at random and thus they present the actual 

 conditions in the population. 



TABLE II. 



It is very clear from the above table that there is a great pre- 

 ponderance of females in Xiphophorus helleri in the immature 

 condition i.e., in the developmental stages falling between 10 and 

 25 mm. in total length. For all practical purposes the ratio may 

 be reduced to two females to one male. 



For sex-ratios in mature fishes the writer is indebted to Dr. 

 A. W. Bellamy. Table III. is compiled from his data. Each 

 record gives the history of an entire brood reared from birth to 

 sexual maturity. 



