THE CYPEINODONTS. 7 



ation, and in the shapes and sizes of their fins are undergone by males in the 

 breeding season, for examples, iu MoUienisia and Xiphophorus, the former in 

 its enlarged and ornamented dorsal fin, the latter in its sword-shaped caudal, 

 and both in the acquisition of brilliant colors over the body. By common 

 experience collectors find males to be less numerous than females. The 

 striking appearance of the male will no doubt be claimed as evidence of 

 selection because of a possible benefit in enabling the female more readily 

 to find him ; it may also be utilized in explaining the discrepancy in num- 

 bers since it must be effectual in making him an object of more prominence 

 and a more frequent prey than the other sex for enemies of the species. 

 From this one might be led to inquire whether the species is not on the way 

 to extinction, or whether the females eventually are somehow to continue its 

 existence on their own responsibility. 



Plates IX. to XII. show admirably, so far as black and white may do it, 

 various phases of the coloration in species of several genera. These plates 

 are from the pencil of the artist Sonrel ; they were originally intended by 

 Professor L. Agassiz for his work on the North American Fishes, of which the 

 present is to be I'egarded as a continuation. The differences between the 

 young stages, in which the sexes are alike, and the adult, in which males differ 

 from females, of Fundulus majalis, are indicated on Plate IX. Very young 

 individuals are blotched with black along the side ; adult males have vertical 

 bands, while adult females are longitudinally banded, except at the base of the 

 tail. On Plate X. variations within the species Zygonectes Nottii are shown 

 to include all phases between such as exhibit a sjjot on each scale and those 

 with longitudinal stripes or transverse bands or combinations of both bands 

 and stripes. Plate XI. depicts, among the ordinary variations of form and 

 coloration, a couple of the startling mixtures occasionally met with in Gam- 

 busia Holbrookii, Fig. 4 and 5, in which the specimen takes on a dress entirely 

 at variance with that connnon to the species. The presence of parasites in 

 certain individuals thus peculiarly marked suggests a possible connection of 

 such variation with disease. The Professor has figured both sexes of Mol- 

 lienisia latipinna on Plate XII., and has indicated the progressive modifica- 

 tion of the male. Ornate as shadow and light have made these figures, 

 without considerable more assistance, the imagination will yet fail to sup- 

 ply the tints, of blue, green, orange, silver and gold, necessary to represent 

 the ornamentation of a living individual. 



The most common form of body is slightly compressed ; it varies to 



