ARCADE. 69 



FAMILY I. ARCADE. 



Shell equal- valved, covered with a dense epider- 

 mis ; hinge forming a large portion of the margin, 

 composed of a great number of similar comb-like 

 teeth, placed side by side ; muscular impressions 

 nearly equal ; pallial line distinct. Animal with 

 the mantle freely open; generally without siphonal 

 tubes ; foot large, deeply grooved, and capable of 

 expanding into a disk. 



Leda (Schum.). Shell oblong, rounded in front, 

 drawn out to a point behind ; margins smooth ; 

 pallial line slightly sinuated. 



L, caudata. Fig. 107 ; n. s. 

 pygmsea. 



Nucula (Lamk.). Shell three-sided, covered 

 with an olive epidermis, pearly within ; margins 

 notched ; pallial line simple ; hinge with prominent 

 internal cartilage pit, and a row of teeth on each 

 side. 



N. decussata. N. radiata. 



nitida. tenuis. 



nucleus. Fig. 108 ; n. s. 



Pectunculus (Lamk.). Shell round, equal-sided, 

 nearly smooth ; teeth arranged in a semicircle. 

 P. glycimeris. Fig. 109 ; f n. s. 



