230 Observations upon the Geology and Paleontology 



without breaking some of its parts. Surface marked by 

 squamose lines of growth, and frequent lines of imbrica- 

 tion, particularly near the margin, proceeding from which 

 occasional specimens show the remains of fringes of con- 

 siderable length. 



This species is somewhat variable, particularly in the 

 convexity of the dorsal valve, and the prominence and 

 curvature of the beaks. The ventral valve is considerably 

 weakened by the large, deep, muscular impression, in con- 

 sequence of which it is liable to be damaged or destroyed. 

 The dorsal valve is stronger, and receives additional 

 strength from its massive hinge-plate, and is consequently 

 more frequently preserved. 



The foramen of the hinge-plate, being so small, is 

 usually filled, and not observable. Width a little greater 

 than the length, seldom exceeding five lines. 



Geological formation and locality. In Nos. 5, 6, and the 

 lower part of No. 7, of the preceding sections, Burlington, 

 Iowa. 



FRODUCTUS L^VICOSTUS (White.) 



Shell of medium size, somewhat ovoid in form, longer 

 than wide, broadly expanding toward the front, moder- 

 ately arcuate above, hinge-line less than the width of the 

 shell in front, ears slightly projecting, strongly wrinkled. 

 Dorsal valve following the curvature of the opposite one, 

 and leaving a moderate space between them. Ventral 

 valve regularly rounded, beak broad, short, slightly in- 

 curved over the hinge-line, strong wrinkles on the sides, 

 extending out upon the ears, which, \\itli ihe broadly 

 rounded umbo, give these parts the appearance of having 

 been pushed forcibly forward. Surface marked by numer- 

 ous filiform costee, of a smooth, wiry appearance, which 

 increase by implantation, as the shell increases in size; 



