NATURAL SCIENCES OF PHILADELPHIA. 323 



SEDGWICKIA? FRAGILIS, Meek. 

 Shell rather small, apparentl}' very thin, loiigitiulinally oblong 

 or siiboval, rather distinctly convex along the umbonal slopes 

 from the beaks toward the posterior basal margin, and down 

 near the anterior side, while just under the beaks a rather strongly 

 marked impression descends, widening and deepening as it ap- 

 proaches the base; basal margin subparallel in its general outline 

 to the dorsal, but diverging more or less posteriorly, where it is 

 most prominent and distinctly sinuons toward the front; poste. 

 rior margin wider than the anterior, and more or less truncated ; 

 anterior extremit}'' ver}' short, and rounded or somewhat trun- 

 cated ; hinge line straight, and shorter tlian the entire length of 

 the valves, apparently very slightly inflected behind the beaks, 

 which are raised a little above the cardinal margin, incurved, 

 contiguous, flattened on the outer sides, and placed near the an- 

 terior end, with a slight forward inclination. Surface ornamented 

 with moderately distinct lines and irregular minute wrinkles of 

 growth. 



The only specimens of this species yet known to me are too 

 imperfect to afford exact measurement, though they seem to have 

 been, when entire and undistorted, about 0.90 inch in length, 0.t3 

 inch in height, and 0.40 inch in convexity. They present some ap- 

 pearance of having been gaping behind and in the anterior ventral 

 region. One specimen looks as if it had been truncated, with a 

 backward obliquity from below upward behind, but this may be 

 due to distortion. 



I am far from being satisfied that this shell is congeneric with 

 the forms for which Prof. McCoy proposed the name Sedgivickia, 

 as nothing can be determined from the specimens yet known, in 

 regard to its hinge and muscular and pallial impressions. Pos- 

 sibl}^ it would be nearer right to call it ModiolopaiH fragilis; but 

 there is something in its physiognomy that suggests affinities to 

 Carboniferous types referred to Sedgwickia and Allorisma. 



Locality and position. — Cincinnati group of the Lower Silurian, 

 at about 350 feet above low-water mark of the Ohio River, at 

 Cincinnati, Ohio. Mr. U. P. James's collection. 



1872.] 



