126 MR. CONRAD'S DESCRIPTIONS 



Differs from the preceding in having a longer hinge, finer radii, &cc., and is a 

 much smaller species and more abundant. 



B. PROTRACTA. PI. 13, fig. 36. — Trapezoidal, elongated, with numerous radiating lines, some of which are 

 double, and others alternated in size and finely crenulated ; dorsal margin parallel with the base ; 

 anterior margin truncated ; posterior a little concave, oblique, end very acutely rounded or subangular; 

 basal margin slightly contracted ; hinge line long, rectilinear, very regular and gradually increasing 

 in width towards the extremities from the apex; cardinal area wide, depressed, concave, with a few 

 fine impressed angular lines. Length I5. Height J nearly. 



A pretty species, of which I found one valve only. 



AVICULA. 



A. ARGENTEA. PI. 12, fig. 10. — Ovate-subquadrangukr, thin and fragile, ventricose above, smooth and 

 entire ? anterior wing sharply angular ; posterior wing not produced, rectangular at the extremity ; 

 posterior end angular, extending beyond the hinge line and much above the line of the base ; anterior 

 margin and base form a regular rounded outline. Height I5, Length 1 4-10. Not common. 



MODIOLA. 



M. MississippiENSis. PI. 12, fig. 19. — Slightly arched, elongated, ventricose, with rather fine closely 

 arranged radiating lines wanting on the anterior side; region of umbonial slope inflated; posterior end 

 acutely rounded ; beak angulated posteriorly ; substance of shell silvery and perlaceous. 2-lOths from 

 beak to base. Rare, except in one spot, where I obtained several specimens and fragments. 



PINNA. 



P. ARGENTEA. PI. 13, fig. 31. — Triangular, with straight margins and acute summit; compressed; 

 substance highly polished and silvery ; valves -with longitudinal radii on more than half the disc, 

 about thirteen in number ; anterior side with rugose, obtuse, oblique, finer and more approximate lines. 

 Length 21. Rare. 



LIMA. 



L. STAMiNEA. PI. 13, fig. 30. — Subovate, oblique, inflated, with fine radiating lines; ears very small, 

 scarcely defined ; posterior margin rectilinear. Height 4-10. 



Very similar in outline to the Miocene species, L. papyria, but it is much smaller, 

 and has more numerous lines anteriorly. Rare. 



PECTEN. 



A valve of a small species was obtained. It is orbicular and entire, and resembles P. calvatus, (Morton.) 



OSTREA. 



O. vicKSBURGENSis. PI. 13, figs. 5, 37. — Plicated ; very irregular and adhering, the upper valve not flat, 

 but swelling in an irregular manner. Height If. 



There is nothing peculiar about this shell, yet it is clearly distinct from any other 

 species of the American Tertiary hitherto described. Common. 



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