Recent Researches of Professor Agassiz. 297 



occurs in the fresh waters of our northern and middle as well 

 as western states, with half a dozen species, some of which 

 have been unfortunately described as Leuciscus, Fundulus, and 

 Hydrargyra, with which genera they have no affinity, while 

 other new ones have been discovered by Professor Baird and 

 myself. I shall call this genus Melanura, from the singular 

 black mark which all species shew upon the tail. But I would 

 tire you were I to go on with my ichthyological remarks, even 

 if I should limit myself to enumerating new genera, for I have 

 many more of these. 



I will close this long letter with one observation upon 

 Crustacea, which may have a more immediate interest for 

 you, if you have not yet noticed the fact yourself. On my 

 return from Florida two years ago, I noticed among many 

 specimens of Lupa dicantha, one in which the tail presented 

 a triangular form intermediate between that of the male and 

 that of the female. Unable to ascertain from a single speci- 

 men whether it was a mere variety, or perhaps an improperly 

 developed female, I awaited another opportunity for a fuller 

 investigation, which the market of Charlestown, S. C, afford- 

 ed largely during the latter part of February last, when I as- 

 certained that that form was at times as common in the mar- 

 ket as either the males or the females, and upon careful ana- 

 tomical examination, I satisfied myself farther, that these 

 specimens are entirely deprived of internal sexual organs, 

 though slight indications of the openings of the sexual organs 

 entirely closed up by calcareous matter, clearly indicated 

 that they are imperfectly developed females, a kind of neuters 

 among crabs, the great number of which leads to the suppo- 

 sition that they are not without function in the general 

 economy of these animals. The tail is soldered to the 

 carapace, the last joints only at which the alimentary canal 

 terminates being movable. At the same time males and 

 females were dissected, and shewed the sexual organs in that 

 fulness which precedes copulation. Looking afterwards for 

 similar conditions in other species, I found in the collection 

 of Professor L. Gibbes, specimens of Lupa cribraria, and of 

 L. Sayi, with the same conformation of their tail. * * * * 

 I cannot help returning to my fishes to say, that I have 



