94: TRANS. OF THE ACAD. OF SCIENCE. 



and flattened towards the cardinal extremities : beak small, 

 strongly incurved over the hinge line; sinus broad, deeply 

 rounded ; cardinal line as long as the width of the shell ; 

 ears large, flat, subacute at the outer angle. Dorsal valve 

 moderately concave, with a small, concave depression under 

 the beak ; mesial fold conspicuous ; ears flattened. Surface 

 marked with numerous, broad, concentric folds, which rise 

 perpendicular in front, forming a sharp, carinated edge on 

 the anterior side of each, and giving them the appearance of 

 steps. The upper surface of each fold is convex and orna- 

 mented with five or six rows of depressed spines — those of 

 the upper row the largest. 



Length, 2.43; width, 2.32; height, 1.03. 



This shell most resembles the P. punctatus, but has a 

 larger sinus, and the concentric bands are carinated in front, 

 and the spines are more depressed, and their bases more 

 elongated on the shell. The P. vittatus is more elongated, 

 more convex and but slightly sinuate ; the markings are also 

 very different. 



This shell is common in the Keokuk Limestone at Keokuk, 

 Iowa, and in Lewis and St. Louis counties, Missouri. 



Productus cor^eformis. 



Shell very small, elongate, ovate or oval, very convex, cos- 

 tate. Ventral valve strongly arched longitudinally ; very 

 convex along the middle from the beak to the anterior mar- 

 gin ; front raised into a rounded mesial fold ; sides descend 

 rapidly to the lateral margins ; ears small, sharply defined, 

 arched, with a few spines on each along the cardinal line ; 

 beak long, pointed, strongly incurved ; cardinal line shorter 

 than width of the shell ; greatest width near the middle. 

 Dorsal valve not known. /Surface marked by tine, rounded, 

 often curved or contorted, radiating costae (increased by im- 

 plantation), and by corrugations on the ears, which often ex- 

 tend up over the visceral region. 



Length, .71 ; width, .51 ; height, .37. 



This shell is easily distinguished from all the American 

 species by its form, and size, and markings. In form it re- 

 sembles the P. magistos, but its ears are smaller in propor- 

 tion, and its size is very much less. It may be a very small, 

 elongated variety of the P. cora. 



From the Archimedes Limestone of Cooper county, Mis- 

 souri. 



Koxinckina Americana. 



Shell very thin, hemispherical, slightly transverse. Ventral 

 valve regularly convex, or slightly flattened in front; beak 

 small, pointed, strongly incurved; auricular appendages 



