296 Recent Researches of Professor Agassiz. 



binations of characters. Several years ago I noticed two 

 species of new genus which I would call Heterandria, from 

 the great difference observed between the two sexes, the 

 males having the ventral fins near the pectorals in about the 

 same position as in the Thoracic fishes, while the females 

 have those fins in the middle of the belly as in the Abdomin- 

 als. Of this genus I have observed several new species. 

 They all live in dense shoals in shallow waters. Another 

 type Zygonectes, presents no such sexual differences, and 

 differs also in its habits. These fishes are constantly seen 

 swimming on the top of the water in pairs, whence their 

 name. I have found half a dozen new species of this genus. 



You may remember the remarkable genus Mollinesia de- 

 scribed by Lesueur from specimens obtained from Lake Pon- 

 chartrain and from Florida. If you do not, pray look for the 

 figures in the Journal of the Acad, of Nat. Sci., vol. ii., to ap- 

 preciate the facts here mentioned. From its structure and 

 from the sexual differences observed among other Cyprino- 

 donts, I have long entertained the opinion that this genus 

 had been established upon the males of Pcecilia mutilineata 

 also described by Lesueur (same Journal), and both are ad- 

 mitted as distinct in the great Natural History of Fishes by 

 Cuvier and Valenciennes. Having found both together in all 

 the Gulf states, I have watched them carefully, and in Mo- 

 bile as well as in New Orleans, I have seen them day after 

 day in copulation during the months of April and May ; so 

 that their specific identity is now an established fact. I have 

 caught hundreds of them and found all the Pcecilias to be 

 females and all the Mollinesias males ; and what is further 

 very interestiug, the females are viviparous. I have been 

 able to trace their whole embyronic development in the body 

 of the mother, in selecting specimens in different stages of 

 gestation. 



I do not remember whether I have already mentioned to 

 you the existence in the United States of two families of 

 fishes not before observed in our waters, one of the Myocinoids, 

 with one species from Eastport in Maine, collected by W. 

 Stimpson, the other the Erythrinoids of Valenciennes, or Cha- 

 raxini without adipose fin of T. Miiller, of which a new genus 



