MANUAL OF NATURAL HISTORY. 159 



1. FAMILY. Scallops (Pectinidse) Mantle open in 



its entire length, with pendent tentacular 

 edges, usually with eye-like spots ; foot small, 

 cylindrical, with a byssal groove shell free, 

 regular, not foliaceous, usually auricled ; liga- 

 ment in a cardinal groove. 



2. FAMILY. Water-Clams (Spondylidse). Mantle- 



margins with truncate cirrhi ; foot short, pe- 

 dunculate, ending in a disk, with an elongate 

 cylindrical tendon arising from its centre ; 

 shell irregular, attached ; hinge with two 

 strong teeth ; ligament internal. 



3. FAMILY. Oysters (Ostreidse). Mantle open in 



its entire length, without siphons, edges 

 double bordered with cirrhi, no conspicuous 

 ocelli ; foot obsolete, rudimentary ; shell irre- 

 gular, attached, foliated ; hinge without teeth ; 

 ligament internal or semi-internal. 



4. FAMILY. Perforated-Oysters (Anorniidse). Mantle 



freely open, with cirrhated margins, no con- 

 spicuous ocelli ; foot rudimentary ; shell emar- 

 ginate, or perforated near the beak, through 

 which opening the adductor muscle passes, 

 attached to an opercular shelly plug. 

 6. FAMILY. Window-Oysters (Placunidse). Shell 

 compressed, thin, transparent ; cartilages on 

 the edge of two divergent ridges on one of 

 the valves, which fit into grooves in the other. 



