MANUAL OF NATURAL HISTORY. 159 



1. Family. — Scallops (Pectinidae), 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 (Spondylidae). 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 (Ostreidae). 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. 



-t. Family. — -Perforated-Oysters (Anomiidae). 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. 



5. Family. — Window-Oysters (Placunidae). Shell 

 compressed, thin, transparent ; cartilages on 

 the edge of two divergent ridges on one of 

 the valves, which fit into grooves in the other. 



