WITTER : UNIO LUTEOLUS AND ITS ALLIED FORMS. 173 



[It will be observed that I have made no mention of the 

 MelaniadcB and Unio7iid(B. At Key West I did not come 

 across a single specimen of these families, which is not sur- 

 prising in the inland torrents, but are fond of fresh water. 

 I found several species in my North American travels, but 

 very few in South Carolina, and those of no particular pecu- 

 liarity of form]. 



My thanks are due to Mr. G. B. Sowerby, junr., for having 

 materially assisted me in the discrimination of the more critical 

 species. It will be observed that the mollusca belong to two 

 distinct provinces : — (i). — The Carolinean section of the Trans- 

 atlantic and (ii) — The Caribbean — The fauna and flora of Key 

 West are almost wholly tropical, presenting a curious change from 

 the mainland of Florida, separated only by a channel of the Gulf 

 stream, some fifty miles or so in width. The Florida Keys, of 

 which this is the principal in size, are entirely of the coral forma- 

 tion, often only a few inches above high water mark. 



UNIO LUTEOLUS, (lamarck). & ITS ALLIED FORMS. 



By Prof. F. M. WITTER. 

 (Read before the Conchological Society). 



Common in slough below Keokuk Lake, Muscatine County. 

 Found in greater or less abundance in the Mississippi at Muscat- 

 ine; in creek near Brown's Ferry &n Cedar river in Muscatine Co.; 

 in Mud creek at Wilton; in Des Moines river at Des Moines; and 

 Skunk river at Ames, in fact it is found almost everywhere in the 

 Mississippi valley, especially in the northern half and in the valley 

 of the St. Lawrence. 



