INVERTEBRATE PALEONTOLOGY. 65 



A few species present iiiy more or less the external appearance of this 

 genus have been described under its name from the Carboniferous rocks ; 

 but their hinges and interior are not well known in some cases, while in 

 others they have been found to belong to Avicula, Monopteria, and other 

 groups. The Permian species referred to it probably all belong to Bakevellia 

 and Avicula. Some of the Upper Triassic species seem to belong to this 

 genus, and it is known to be well represented in the Jurassic rocks, during 

 the formation of which it probably attained its greatest development. It 

 continued also to be represented until about the close of the Cretaceous 

 period, after which we find no traces of its existence. 



Ocrvillia sutttortuosa, M & H 



Plate 16, tigs. 7, (i, b, c. 

 Gertnllia siMortiiosa, Meek ami Hayden (Nov., 1856), Proceed. Acad. Nat. Sci. Pliilad., VIII, 276. 



Shell thick, obliquely sublanceolate, tortuous, and laterally curved ; right 

 valve distinctly convex, left nearly flat; posterior side elongate, narrow, widest 

 at its junction with the posterior extremity of the hinge; cardinal border 

 straight, rather long, and forming an angle of about 20° witli the longitudinal 

 axis of the shell; cardinal area rather broad; cartilage-pits about six, nearly 

 as wide as the intervals between; hinge-teeth nearly obsolete; scar of the 

 adductor and posterior pedal muscles large, obliquely elongate, and placed 

 nearly centrally above the middle of each valve; anterior scar small, very- 

 deep, and located close under the anterior extremity of the hinge. Surface 

 unknown. 



Our specimens of this species an 1 imperfect at both extremities, and the 

 external subcorneous layer of the shell is exfoliated; consequently they do not 

 afford the means tor making out a very complete diagnosis. It is evidently 

 quite a thick shell ; the remaining portion after the exfoliation of the outer 

 layer and some of the nacreous lamina?, measuring in places as much as 0.34 

 inch in thickness On the left, or more convex, valve, we observe an unde- 

 fined suclus passing obliquely from the anterior side of the beak backward 

 and downward to the lower margin, which it intersects nearly under the 

 middle of the hinge. At this point, the lower edges of both valves are a 

 little contracted, and, just in advance of it, they are slightly gaping for the 

 passage of the byssus. 

 9 ii 



