309 BuLivKTiN 5 39 



FOSSIL SHELLS. 133 



subovate : inferior valve convex, with numerous 

 convex ribs interrupted b}' fornicated scales at 

 the lines of increment ; hijige curving a little up- 

 ward, very much contracted and short : superior 

 valve flat, wrinkled concentrically, without any 

 appearance of longitudinal lines : hi^ige more di- 

 lated than that of the superior valve, and oblique 

 with respect to the thickness of the shell. 



Length of the specimen four inches and four- 

 fifths ; breadth four inches and a quarter. A large 

 superior valve is five inches and three quarters 

 long, and five inches and a quarter broad. 



Ivike many species of this genus, it varies much 

 in form, and in the prominence of the ribs on its 

 convex valve ; but these ribs are very obvious on 

 the nine specimens under examination. Exter- 

 nally some varieties have a striking resemblance 

 to O. bellovacina, Lam. but the hinge is much 

 more contracted. 



Besides those obtained by Mr. Finch, Mr. Z. 

 Collins presented a fine specimen to the Acade- 

 my, found on the west branch of the Potomac, 

 about fifteen miles below Alexandria. 



PBCTKN. 



P. Jeffersonius. PI. ix. fig. i. Subequivalve, 

 with from nine to eleven striated ribs. 



Description. Shell rounded, convex, not quite 

 equivalved, one of the valves being a little more 



[J. A. N. S., Phila., ist Ser., Vol. IV, 1824.] 



