82 TRANS. OF THE ACAD. OF SCIENCE. 



Ortiiis Cooperensis. 



Shell small, orbicular or subquadrate, widest at or below 

 the middle, cist ate. Ventral valve convex towards the beak, 

 depressed or flattened towards the margins, with a broad 

 shallow sinus in front; beak long, strongly incurved; area 

 small ; hing< line one third as long as the width of the shell. 

 Dorsal valve more convex, flattened towards the margins ; 

 umbo full; beak shorter, curved up nearly in contact with 

 that of the opposite valve. Surface marked with tine, radia- 

 ting costae (increased by implantation), and with fine concen- 

 tric striae. 



Length, .51 ; width, .49; thickness, .21. 



This fossil was collected by Dr. Giddings from Warsaw 

 Limestone in Cooper county, and by Dr. Norwood, at Bar- 

 rett's Station, in St. Louis county, Missouri. 



Orthisina occidentalis. 



Shell large, very gibbous, transverse, wider below the 

 middle, plicate, and decussate. Ventral valve very convex, 

 highest towards the beak, descends nearly perpendicularly 

 towards the cardinal extremities, which are slightly auricu- 

 late; margin broadly rounded or slightly concave in front, 

 rounded towards the cardinal extremities; area slightly con- 

 cave, triangular, sides shorter than the base; deltidiurn nar- 

 row, prominent, rounded ; umbo depressed ; beak small, 

 pointed ; cardinal line much shorter than the width of the 

 shell. Ventral valve less convex, subelliptical ; umbo full, 

 projecting beyond the cardinal line; beak abruptly incurved 

 to the cardinal line. 



Length, 1.39; width, 1.75; thickness, 1.16. 



Anterior margin of each valve marked with about ten 

 broad, depressed plications, which disappear before reaching 

 the middle of the shell. Whole surface ornamented with 

 small radiating costae (increased by implantation), and by 

 tine concentric stme or raised lines; and also by very dis- 

 tinct plications of growth near the margins. The umbo of 

 the ventral valve has a few indistinct ruga?. 



This shell most resembles the 0. Jlitssow'iensis / but the 

 ventral valve is not so high, area is wider and not so high, 

 plications are more depressed and confined to the margins, 

 and the markings on the plications are not diagonal. It is 

 more convex than the O. Shumardiand ; its radiating costaa 

 are much smaller, and not striate as in that fossil. 



This fossil was discovered in the Upper Coal Measures of 

 Caldwell county, Missouri, by Mr. Wheeler. 



