262 TRANSACTIONS OF THE [MAR. 15, ' 



on the central portion of the shell, and with concentric lines of 

 growth. 



All the specimens of this shell in the collection are imperfect, 

 so that the marginal contonr has been determined chiefly b}- the 

 lines of growth. One specimen shows both right and left valves 

 partially open, and it is the left valve of this specimen which 

 has been figured. It does not resemble any of the heretofore 

 described American species of the genus. 



The species is named in honor of Mr. T. C. Hopkins, a mem- 

 ber of the late Geological Survey of Arkansas, who did much 

 valuable work on the stratigraphy in northern Arkansas, 



Pteronites l.evis sp, nov. 

 PI. XX., figs. 11-12. 



Shell subovate in outline, convex anteriorly, moderately de- 

 pressed posteriorly. Posterior wing ill-defined, not sharply 

 seperated from the body of the shell. Beaks terminal. 



Hinge-line straight, shorter than the most posterior extension 

 of the shell below. Antero-ventral margin forming a semi-oval 

 curve, bending regularly upward posteriorly, into tUe posterior 

 margin which meets the cardinal line at an oblique angle. 



Surface smooth except for a few indistinct lines of growth 

 which are often obsolete. 



In general form this species is similar to P. hopkinsi, but 

 differs from that species in its smaller size and in its unorna- 

 mented shell. 



Myalina arkansana sp. nov. 

 PL XX., figs. 16-17. 



Shell below medium size, oblique, the greatest length about 

 equal to twice the greatest width. Valves strongly convex in 

 the umbonal region, compressed posteriori}'. Beaks depressed, 

 in the younger specimens almost terminal, in the older ones at 

 a little distance from the anterior extremity. A shallow de- 

 pression extends from the beak obliquely to the ventral margin 

 in front of the unbonal ridge. 



Hinge-line straight, equal to about two-thirds the entire 

 length of the shell. Anterior margin obtusely rounded, ventral 

 margin slightly arcuate, regularly rounded upward posteriorly 

 into the posterior margin, which curves forward in an arcuate 

 line or is nearly straight, meeting the hinge line at an obtuse 

 angle of abont 1.35°. 



