14 



fades away below and is lost in the geneial convexity of the shell. 

 The wing is small, flat aud has au acute extremity. Shell marked 

 with distant concentric lines of growth. 



All of our specimens are casts of the left valve, no part of the 

 shell is preserved, and no radiating lines are disceruable. 



This species is distinguished by its oblique, elongate-ovate form 

 and small posterior wing. 



Found by Miss Jessie Blair, in the Chouteau limestone, near 

 Sedalia, Missouri, and now in the collection of S. A. Miller. 



Family MYTILID.E. 



MYT-ILA.RCA JESSIES, D. sp. 



Plate IF, Fig. J, left valve nearly complete; Fig. 2, another left 

 ratre with more of the anterior end broken off; Fig. 3, part 

 of another left valve; Fig. 4, anterior end of right valve 

 complete; Fig. 5, anterior end of left valve complete, 

 small specimen j Fig. 6', part of the surface 

 of the shell magnified four diameters. 



Our specimens are quite variable, from medium to considerably 

 above medium in size. 



Shell oblique, elongate, subelliptical, with venticose valves in 

 middle part, where it is subcylindrical. The diameter through 

 the valves in the middle area, is nearly equal to the distance 

 from the hinge to the basal margin. Posterior end of the shell 

 cuneiform and narrowly rounded. Basal margin of the shell 

 oblique, nearly straight or slightly arcuate. Hiuge line about 

 half the length of the shell, slightly arching. Posterior to the end 

 of the hinge, the shell is slightly arcuate until it adruptly curves 

 into the post-basal margin. Beaks obtuse aud terminal at the 

 anterior end. Umboaes high and abrup ly rounded to the hinge 

 line, posterior to which the umbonal slope merges into the cuneate 

 form of the shell. Surface marked by concentric lines of growth. 

 Shell thin. 



This species is readily distinquished by its general elongate 

 form from all others. We have numerous fragments of this spec- 

 ies but most of them are cast?. Part only of the shell is preserved 

 on a few of the fragments. 



