l60 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



ally easily distinguished by the presence of a delthyrium or pedicle 

 opening through which the pedicle is protruded. In many Atrema- 

 tous genera the ventral valve is not readily distinguished. When the 

 shell is cemented to foreign bodies it is always by the ventral valve. 

 It is usually the larger and deeper of the two valves. Pedicle, 

 larger, dental, neural, and receiving valves are synonymous terms. 



Vascular' (Pallial) Sinuses. — Two convergent or divergent 

 primary sinuses of the circulatory system, traversing the mantle and 

 originating in the posterior medial region. They usually have 

 numerous secondary (lateral and peripheral) branches and both 

 often leave impressions in the shell. 



Visceral Area. — The posterior region of the interior of the 

 valves between the pallial sinuses ; in general, the immediate area of 

 the median muscle tracks. 



Visceral Cavity = Visceral area. 



