60 AMERICAN CORBICULAD^E. 



sjphonal tube ; oral aperture small, tentacles of the mouth tri 

 angular, elongated ; gills of medium size ; foot small, tongue- 

 shaped, capable of great extension. 



Shell small, rounded-oval, inequilateral, anterior side longer ; 

 beaks terminal ; cardinal teeth double, at times united, situated 

 immediately under the beaks ; lateral teeth elongated, lamelli- 

 forni, double in the right valve, single in the left valve ; ligament 

 always on the shorter side. 



This genus was instituted by Pfeiffer for a class of shells which, 

 by the older authors, had been confounded with Tellina, and more 

 recently with Sphserium. The differences which exist between 

 Pisidium and Sphserium, which led Pfeiffer to establish this new 

 genus, are very material, both in the animal and in the shell. The 

 animal of Pisidium has the lobes of the mantle united posteriorly 

 into one syphonal tube ; whereas in Spheerium, the lobes form a 

 tube, single at its base, but double at the extremity. According 

 to M. Deshayes there is much difference in the formation of the 

 foot of Pisidium from that of Sphderium that of the former 

 being somewhat similar to that of Cardium. This organ is 

 capable of great extension, and when prolonged to its utmost 

 extent, is not unlike that of Lucina ; it is made use of much 

 in the same manner as that of Sphserium, either to burrow into 

 the ground or to crawl on solid bodies. 



With regard to the shell, the main difference consists in the 

 position of the beaks. In Pisidium the beaks are terminal, that 

 is, the distance from the beaks to the anterior extremity is greater 

 than the distance from the beaks to the posterior extremity. In 

 Sphserium, on the contrary, the position of the beaks is generally 

 central, dividing the hinge-margin into equal portions. The 

 formation of the hinge of Pisidium is very similar to that of 

 Sphserium; the position of the cardinal teeth, however, in Pisi- 

 dium is terminal, like that of the beaks, whereas in Sphserium it 

 is central. The teeth of Pisidium are, in proportion to the size 

 of the shell, more robust than in Sphserium. 



The habits and mode of living of the species of these two genera 

 are very much the same. They are found buried an inch or more 

 in the mud under water, or else attached to the roots or stems of 

 aquatic plants. 



