PANAMIC-PACIFIC PELECYPODA 223 



bored by marine organisms may prove difficult to determine. Recent mem- 

 bers of the family belong to three groups, usually classed as genera. In the 

 typical or dextral Chamas, the attachment is by the left valve (but not 

 invariably so), hence, the upper or free valve seen from the inside, 

 shows the beaks directed or coiled towards the right. In PseudochaiTui, at- 

 tachment is by the right valve, and the beaks are sinistral or coiled towards 

 the left. In Echinochama, the valves are attached only in the early stages 

 by a small area along the anterior side of the right valve; its valves are, 

 therefore, nearly alike in shape and sculpture and provided with a deep, 

 cordate lunule defined by an incised line. As in other attached mollusks, 

 the regularity of the sculptural pattern, even that seen on the upper or 

 free valve, is determined to a large degree by the size and irregularity of 

 the substratum to which the shell was attached, thus the normal sculptural 

 pattern is generally not assumed until the growing edge of the lower valve 

 has become free. 



Key to genera and species of Chamidae 



I. Valves are nearly alike in shape and sculpture. Attachment is temporary, 

 the shell becomes free and loose in the adult, the attachment scar hence 

 small or obsolete. There is a large, cordate lunule outlined by an incised 

 line. 



Genus Echinochama 



1. Sculpture ribbed and bearing large, sharp spines. 



E. arcinella californica 

 II. Shell permanently attached, the attachment area large. Strongly in- 



equivalved. 

 A. Shell attached normally by its left valve, the beaks directed or coiled 

 towards the right. The dextral Chamas. 



Genus Chama 



a. Outer shell layer showing as a wide band around the inner margin of 

 the valve. 



aa. Interior of the valves white. 



2. Shell of moderate size, usually rounded. The outer surface and its 

 sculptural elements have a waxy or translucent luster. 



C. pellucida 



3. Shell small, rounded, and covered with short white spines. 



C squamuligera 



4. Shell relatively small, the lower valve convex with strong, concentric 

 sculpture; the upper valves with a scattering of small spines. Color red. 



C. sordida 



ab. Interior of shell more or less colored. 



5. Shell relatively large, its surface with an elaborate pattern of yellow 

 or purple, foliated spines. Inner margin of shell with a purple border. 



C. frondosa 



6. Shell smaller, its surface bearing small, close-set spines. Color purple 

 or coral-red, the cardinal tooth always colored deep red. 



C. echinata 



b. Outer shell layer thin, the inner marginal band hence narrow. 



7. Shell often large, its surface with a ground color of brownish violet, with 

 radial rows of short, white spines. 



C buddiana 



