Fara. 2.] NATURAL HISTORY SOCIETY. 85 



CLASS I. PELEOYPODA. 



Laterally compressed, headless Mollusks encased in a bivalve 

 shell. Their organs are inclosed in a mantle, one leaf of which 

 envelops each side. Just within the mantle there are in most 

 cases four, leaf-like, striated gills, two on each side. The shell 

 consists of a right and left valve, with more or less of a dorsal 

 hinge, opened by means of a self-acting, elastic substance, and 

 closed by either one or two muscles which extend between the 

 valves. The sexes are usually separate. 



Order PRIONODESMACEA. 



Hinge teeth almost entirely transverse to the hinge margin, 

 rarely few, often numerous ; substance of the shell often in great 

 part pearly; very few are entirely limy or porcellanous. 

 Family 1. OSTREID/E. 



Shell inequivalve, slightly inequilateral, free or adherent, rest- 

 ing on one valve ; umbo central, not oblique ; ligament internal ; 

 epidermis thin ; adductor impression single in the adult, behind 

 the centre ; pallial line obscure ; hinge usually without teeth. 

 Animal marine ; mantle fully open, slightly adherent to the edge 

 of the shell ; foot small or absent ; gills crescent shaped, two on 

 each side. 



1. OsTREA, L. The Oysters. 



Shell attached by the left valve ; the upper valve flat or con- 

 cave, often plain ; lower convex, often plaited or foliaceous, and 

 with a prominent umbo ; cartilage pit elongated ; hinge toothless ; 

 substance somewhat pearly, laminated. 



1. Ostrea Virginicd, Gmel. Shell gradually widening from 

 the long, pointed umbo; surface, when not worn, showing leaf 

 like scales; muscular impression large, distinct, and near the 

 centre. Surface leaden color ; shell whitish ; muscular impres- 

 sion chestnut or violet color. Distance from umbo, 150-250 ; 

 L. 70-80. — Whole coast. Abundant. 



Family 2. ANOMIID/E. 



Shell thin, pearly, with a deep notch or hole in the under valve 

 near the umbones, for a byssal plug by which the shell is at- 

 tached. 



Animal without palpi around the mouth ; mantle open except 



