PISIDIUM. 61 



Their mode of breeding is much the same, both ejecting the 

 young when sufficiently mature, which up to that time they carry 

 between the fokls of the gills. Pfeiffer supposes the Fisidium to 

 breed by throwing out eggs, but I have myself found young in 

 the shells of Pisidium. 



The most appropriate time of the year for collecting Pisidium, 

 in the North, would seem to be from the middle of April to the 

 early part of July — the season during which they breed ; some 

 species, however, such as P. variabile, I have found at all seasons, 

 even in winter ; others, such as P. ventricosum, are seldom found 

 but in the early summer. Live specimens may be preserved for 

 examination, for some time, if the water is kept sufficiently fresh. 

 On the application of water, slightly warmed, they exhibit great 

 activity, extending their syphonal tube and foot. They not only 

 crawl on the sides of the vessel, in which they are confined, but 

 also on the under surface of the water. They are very similar in 

 some of their movements to certain species of fluviatile Gaateroprxh. 



While this genus has of late years been generally adopted by 

 the Conchologists of Europe, and more recently by those of this 

 country, some English authors have committed the error of apply- 

 ing to it the names of Pisum and Musculium — a great careless- 

 ness on their part, as the types of these two genera, as set out 

 by their respective describers, Megerle and Link, are genuine 

 Sphaeria. 



The genus Pisidium is very abundantly distributed over both 

 sections of this continent ; and while the species of America are 

 entirely distinct as such from those found elsewhere, their forms 

 present, in nearly every instance, great analogies with those of 

 the species of Europe and of Asia. 



1. Pisidium virginicum, BouEGuiGNAr. — Shell large, thick, 

 oblique, very inequilateral ; anterior side longer, nar- 

 rower, rounded ; posterior broader, subtruncate at end, 

 basal margin rounded ; beaks situated posteriorly, large, 

 prominent ; valves solid, moderately convex, interior 

 light blue ; striae coarse and irregular ; epidermis green- 

 ish-brown or chestnut color, with zones ; hinge-margin 

 verv much curved ; hinge broad, two strong cardinal „ . . . 

 teeth, disposed in the shape of the letter V reversed ; 

 lateral teeth strong, short. 



Long. 0.35 ; Lat. 0.29 ; Diam. 0.21 inches. 



