40 . TRANS. OF THE ACAD. OF SCIENCE. 
bous, produced and abruptly attenuated pcnivrionlay - extremity 
truncate; umbones prominent, incurved, that right.valve 
situated in advance of the other; surface polished, (ae marked 
with numerous, closely arranged, fibrous, concentric striz, a 
: poner 44 lines; height, 33 lines; ‘thickness, 22 lines. . 
'  _Locality.—In the septaria of the Cretaceous Formation, of Sage | 
Creek, Nebraska Territory. 
Myritvus Merxit. 
Shell ovate, subquadrate, transverse, height equal to via walk 
the length cardinal margin slightly arcuate ; umbones v 
lique, convex ; heaks situated near the posterior extremity; Rite 
ed, rather obtuse, rioderatély prominent; buccal margin short, 
strongly rounded; anal margin expanded and regularly “rounded 5 
ferior margin nearly straight; surface with obscure concentri¢ 
fold most pees posteriorly; test very thin. .. 
largest specimen, 94 lines; height, 5 fined thick- = | 
madly diathighislied from M. Galpinianus (Evans & Shu=- 
_ mard) by its smaller size, the greater gibbosity of the beaks, and 
its subquadrate form. It can scarcely be confounded with J 
__attenuatus (Meck & Hayden), which is a much more slender — 
y—Moreau River. 
e Osenza 5 SUBTRIGONALIS. 
