THE NAUTILUS. 105 



able, and it has been used long ago by some writers, e. g. for 

 the ' ' lateral teeth ' ' of the Naiades. 



It may appear somewhat surprising that the hinge of Pisi- 

 dium, as a whole, is generally better formed, and much more 

 diversified, than those of the other genera. Even in mussels 

 two millimeters long or less when full grown, the hinges are 

 perfect. But there are considerable, or extreme, differences 

 as to the general shape, and the configuration of the several 

 parts, as exemplified in pi. 3, figs. 4-6. 



In order to simplify description, symbols that is, letters 

 and numbers --have been used by F. Bernard and others, 

 especially for the cardinals and lamella? ; see pi. 3, figs. 1 to 3 

 and explanation. 



THE RIGHT CARDINAL. It has been stated by earlier writers, 

 e. g. Baudon, Prime, Clessin, Westerlund, that in the right 

 valve of Pisidium amnicum Miiller and dubium Say 1 there 

 are two cardinals, and a group, or subgenus, has been estab- 

 lished principally on the strength of that feature. But it was 

 a misconception, due to careless examination; in adolescent 

 and mature mussels, C3 is more or less deeply emarginate 

 in the middle, and the anterior and posterior parts were 

 taken for two teeth. (See Sterki, 1. c.) It should be noted 

 that in some other species of Pisidium the crest of C 3 is more 

 or less emarginate, and also generally and markedly so in 

 Musculium. 



This mistaken view has been applied indiscriminately to all 

 species of Pisidium by some authors, e. g. T. Prime, and even 

 to all Sph&riida: (Prime, '65, pp. 2, 33, 36 etc.), and it was 

 copied, evidently without examining a specimen, by some 

 American writers, e. g. E. E. Call (1900, p. 438 etc.). 



While in the sense pointed out the right cardinal is single, 

 its posterior part is more or less distinctly complex in most 

 species, and also in Sphccrium and Musculium. In many de- 

 scriptions it has been stated that in Pisidium the posterior 



1 Generally known as virginicum (Gmelin) Jenyns; but its identity 

 lias been doubtful. Becently, upon careful investigation, Dr. Pilsbry has 

 restored T. Say's name dubium (Cyclas dubia*). 



