THE NATURAL HISTORY SURVEY. 125 
Pisidium differs from Spherium principally in its siphonal 
tubes, which are united in the former for their whole distance, 
and in the latter are united only at the base. The foot also 
differs, being similar in some respects to Cardium. (Prime.) 
The beaks in Pis¢dium are placed near the fosterzor end, while 
in Spherium they are placed centrally or near the anterior end. 
Pre? lil 
Cardinal teeth of Pisipr1umM COMPRESSUM Prime (x 40, original). Upper 
figure, right valve; lower figure, left valve. 
The mode of reproduction is the same in both genera. Pisidia 
may be more profitably collected during the spring and sum- 
mer, “from the middle of April to the early part of July,’’* this 
being the breeding season. 
The cardinal teeth are sometimes quite similar to (Fig. 11) 
and again quite different (Fig. 12) from those of Spherium. The 
Bre. 12: 
Cardinal teeth of Pisip1um virGinicuM Bourg. (x 50, original). Upper figure, 
right valve; lower figure, left valve. 
teeth in Pistdium are generally more elongated than those of 
Spherium. In one type (Fig. 11) the single cardinal in the right 
valve is long, narrow and curved, the lower part of the curve 
being thicker than the upper part, and placed near the ventral 
border of the hinge plate. The upper tooth in the left valve is 
*Prime, Monograph, p. 6r. 
