SPH^RIUM.. 38 



the largest ; the palleal impression is parallel with the basal mar- 

 gin ; it is narrow and always simple. The hinge-margin is very 

 variable ; it is usually composed of two small teeth in each valve ; 

 at times, however, they are single in one and double in the other, 

 or else single in both valves ; these teeth are occasionally rudi- 

 mentary, or even nearly obsolete. The lateral teeth placed on 

 each side of the cardinal teeth are double in the right valve and 

 single in the left one ; the anterior lateral tooth is usually the 

 shorter. The ligament is external ; it is short, not very con- 

 spicuous, and is always found on the longer portion of the shell. 



The animal of Sphaerium has a broad foot, capable of conside- 

 rable extension ; it uses it either to bore holes in the mud, in 

 which it sinks the posterior portion of the shell, or as means of 

 locomotion. The syphoual tube is double and very retractile ; it 

 is often white like the foot, but at times it is colored. 



The habits of these molluscs are very similar to those of Fisi- 

 dium, with which they are often found living. The species of 

 Sphaerium are less abundant in individuals than those of Fisi- 

 dium ; they are also less generally distributed, and arc more con- 

 fined to certain localities than the latter. 



a. Beaks rounded, never TUBEECtTLAR. 



1. SpliaeriHiii sulcatlinij Lamarck. — Animal white, tubes a 

 ligiit orange color. 



Shell transversely oval, nearly equi- F^g- 25. 



lateral, light in texture for its size ; 

 posterior margin somewhat more point- 

 ed ; anterior rounded, base slightly 

 curved ; valves convex ; beaks full, 

 raised above the outline of the shell; 

 posterior portion a little longer ; sulca- 

 tions coarse, regular; epidermis dark 

 chestnut-brown ; interior light blue ; 

 hinge-margin narrow, nearly a straight 

 line; cardinal teeth small, indistinct, 

 situated somewhat towards the anterior Sph. sulcatum. 



side, double in both valves, and so 



placed as to assume the shape of the letter V reversed ; lateral teeth on a 

 line with the primary teeth, large, strong and prominent. 



The young is more equilateral than the adult ; more compressed ; it pre- 

 sents the shape of a quadrilateral, it is of a light lemon color, the striations 

 are as heavy as those of the mature shell. 

 3 



