Ceetaceous period. 173 



by a large scar of attachment ; umbonal half obtusely carinate, the sides 

 sloping abruptly from the carina to the margins ; basal half not so deeply, 

 but more regularly convex than the other. Test very massive, sometimes 

 ha^'ing a solid thickness of five or six centimeters, lamellose, so much so 

 that the valve often splits into numerous pieces along the surfaces of the 

 layers of growth ; inner surface smooth ; muscular scar of moderate size, 

 somewhat deep, placed about midlength of the valve, and, as usual, a little 

 nearer to the posterior than to the anterior side ; surface marked by strong, 

 irregiilar, imbricating lamellae of growth, which become laciniate at and near 

 the margins ; svirface also marked by fine concentric strise, and by irregular, 

 indistinct, radiating cost£E, the latter being usually removed by exfoliation 

 from old shells. The collections do not contain any example of the upper 

 valve, but both Roemer and Conrad describe it as thick, concentrically 

 laminated ; smooth within ; umbo horizontal, distinctly spiral. 



Length of an example rather under the average size, from umbo to 

 basal margin, about one decimeter: breadth, eight centimeters; convexity 

 of the larger valve, nearly six centimeters. 



Among the numerous examples of this species in the collections, none, 

 except the one figured, show the radiating costae, and these costse seem to 

 be quite different from those, at least of the typical forms, of E. costata Say. 

 Mr. Conrad states, however, that in New Jersey, Alabama, and Texas every 

 intermediate gradation of form and character is found, from typical forms of 

 -E. costata to E. ponderosa. Judging from our examples alone, no person 

 would suspect such specific relationship ; and, in want of any intennediate 

 forms for personal examination, I prefer at present to place our examples 

 under the designation given by Dr. Roemer. 



Position and locaUti/. — Strata of the Cretaceous period; east of Impracti- 

 cable Ridge, Utah. 



Exogyra laeviuscula Kcemer. 



Plato XVII, lig. 2 a, b, c, and d. 



Exogyra IwviuscuJa Ecemer, 1852, Kreidebildung von Texas, 70. 

 Exogyra Iccviiiscula Courad, 1857, U. S. & Mex. Bound. Surv., i, 154. 

 Gryphea Iccviusctila Oonrad, 1857, ib., p. 170 and pi. vii. 



Shell of moderate size, capacious, somewhat semi-ovate in form, sub- 

 orbicular in marginal outline ; test not massive ; lar^-er valve much inflated 



