THE NAUTILUS. 



in producing differentiation in a single type of shell. P. levettei, 

 chiricahuana, ashmuni, pseudodonta and rhyssa are obviously off- 

 shoots of a common stock. 



CONCHOLOGICAL NOTES FROM LOUISIANA. 



BY LORRAINE S. FRIERSON. 



Being, so far as I know, the only student of conchology in Louis- 

 iana, perhaps a few notes may be of interest. 



My station consists of an arm of Red River (Bayou Pierre), a 

 lake and numerous creeks. 



In these waters occur about 50 species of Mollusca, of which 30 

 are Unionidae. An interesting fact, and one which I have never 

 seen mentioned, is the sharp line of separation between the forms 

 found in the creeks and those growing in the Red River waters. 



In the creeks are found 5 Unios and 2 Anodontas. While in the 

 Red River waters are found 25 Unionidse. No creek shell grows in 

 Red River waters (with but two exceptions noted below) and no 

 species living in Red River waters are ever found in the creeks. 



At the junction of a creek with the river occurs a zone where no 

 mollusca can be found. The exceptions noted are U. texasensis Lea, 

 which, while pre-eminently a creek shell, is found sparingly in Red 

 River waters. Another possible exception is in the case of U. de- 

 dims Say. This shell is found abundantly in the creeks, and grows 

 to a large size. A rather rare shell is found in the Red River waters 

 which is said by our authorities to be a form of U. declivis Say 

 known as tetralasmus Say or geometricus Lea. 



Declivis proper never occurs in the Red River waters, nor does 

 tetralasmus ever grow in creek waters. Perhaps this fact will help 

 show that these shells are really distinct species and not synonyms. 



The most variable and, perhaps, the most abundant Unio is U. 

 multiplicatus Lea. West of the Mississippi drainage this shell 

 merges into U. eightsii Lea, and here, at the middle ground, a mix- 

 ture of types is seen. An interesting shell is sparingly found here, 

 close to its extreme southern limit, probably. It is a dwarfed, 

 almost " run out " form of U. donaciformis Lea. Another " Yankee 

 down south " is a very small but brilliantly colored Anodonta sub- 

 orbiculata Say. It is found in the soft, deep mud of Edwards' Lake, 

 and disputes its territory with Anodonta virens Lea. A. imbecilis 



