THE NAUTILUS. 75 



EULIMA COSSMANNI n. Sp. 



M. Cossraann has figured in " Notes Complementaires," PI. I, fig. 

 34, page 27, 1893, as Rissoina notata Lea, a form that is not the 

 species of Lea but a new form ot JSulima, which may be called 

 E. cossmanni. The shell described by Lea is also a Eulima, 



The description and figure given by M. Cossmann are accurate for 

 this new species. The true species of Lea also has a sinuous outer 

 lip. 



Dr. Paul Bartsch has examined specimens of the true Pasithea 

 elegans H. C. Lea, and finds it to be a Bittium. 



NOTES ON TRUNCILLA, WITH A KEY TO THE SPECIES. 



BY BRYANT WALKER. 



As the highest expression of Unione development, the Truncilla 

 are of special interest to the systematic conchologist. Not only art; 

 the sexes sharply differentiated in all the species, but the species 

 themselves are more clearly defined and less subject to variation 

 than in any other of the recognized genera. 



For this reason, the species are well adapted to the rigid limita- 

 tions of a key, which in the more variable groups would, in many 

 cases, be almost impracticable. But in genera such as this, where 

 the specific lines can be drawn with sufficient exactness for such a 

 purpose, the formation of a key, besides facilitating the identification 

 of the species, is of great service in developing the peculiar dis- 

 tinguishing characters of the different species, and thus determining 

 their proper position in a natural arrangement. 



In attempting to make a key to the species of Truncilla, it almost 

 immediately became obvious that, owing to the extreme differentia- 

 tion of the sexes, which very often was not along analogous lines in 

 species of the same group, a single key including both sexes was not 

 feasible, and accordingly a separate key for each sex was made. 



This condition also demonstrated that a consistent natural arrange- 

 ment of the species would have to be based primarily on the varia- 

 tions of one of the sexes. 



In view of the fact that the most recent classification of the 

 Unionidce is based primarily on the modifications of the gill of the 



