Logan.] The Invertebrates, Benton Group. 447 



mental groove deep and containing numerous cartilage pits ; 



test thick, thicker on hinge side. Length of longitudinal axis, 



seventy mm. ; maximum breadth, forty mm. ; thickness of test, 



four to ten mm. 



Both valves of this form imbedded in a firm matrix of Lincoln 



marble were found about fifteen miles southwest of Beloit, on 



the Buel ranch. In the same stone are the impressions of shells 



which resemble Mocliolae in form and markings. There are also 



fragments of other shells which resemble Ostrea subspatulata, but 



which I judge to be Ostrea congesta. I have found these shells 



in the Lincoln marble horizon only, and they are not abundant 



there : 



Exogyra ponderosa Boemer. 



Exogyra ponderosa Boemer, 1852, Kreid. von Texas, p. 71, pi. ix, ff. 2a, 

 b : White, 1S76, U. S. Geog. and Geol. Surv. West 100th Mer., vol. iv, p. 172. 

 pi. xiv, ff. la-c; 1884. 4th Ann. Bep. U. S. Geo). Surv., p. 306, pi. l, ff. 1, 

 2, 3; Stanton, 1893, Bull. U. S. Geol. Surv., p. 65, pi. vn, ff. 1, 2. 



Exogyra costata var. Conr., 1857, U. S. and Mex. Bound. Surv., vol. i, p. 154, 

 pi. vni, f. 3, pi. ix, f . 1. 



Exogyra fimhricata Conr., 1857, id., pi. vn, ff. 2a, b. 



"Shell large, capacious; marginal outline irregularly sub- 

 ovate ; larger valve very gibbous ; umbo distinctly spiral, but 

 the coil is usually obscured by a large scar of attachment ; urn- 

 bonal 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, some- 

 times having a solid thickness of five or six centimeters, la- 

 mellose, so much so that the valve often splits into numerous 

 pieces along the surfaces of layers of growth ; inner surface 

 smooth ; muscular scar of moderate size, somewhat deep, placed 

 about mid-length the valve, and, as usual, a little nearer to the 

 posterior than the anterior side ; surface marked by strong, ir- 

 regular, imbricating lamella3 of growth, which become laciniate 

 at and near the margins ; surface also marked by fine concen- 

 tric strite, and by irregular, indistinct, radiating costse, the lat- 

 ter being usually removed by exfoliation from old shells. Upper 

 valve thick, concentrically laminated ; smooth within ; umbo 

 horizontal ; distinctly spiral ; length of an example rather under 

 22— iv 



