Annual report of the United States Geological Survey to the Secretary of the Interior . average size and «. Ventral view, showing slightly twisted and overarching apex. The stria? and the sinus are both seen to be faintly denned. 36. Posterior view of same. The rounded character of junction of theareal plane with the sides of the shell is well shown. This is con-stant and characteristic. 3c. Anterior view of the same. 3rf. Side view, in which the height of the ventral valve and the slightconcavity of the area are well limestone; Engineer Mountain 7. 4. A small and imperf
Annual report of the United States Geological Survey to the Secretary of the Interior . average size and «. Ventral view, showing slightly twisted and overarching apex. The stria? and the sinus are both seen to be faintly denned. 36. Posterior view of same. The rounded character of junction of theareal plane with the sides of the shell is well shown. This is con-stant and characteristic. 3c. Anterior view of the same. 3rf. Side view, in which the height of the ventral valve and the slightconcavity of the area are well limestone; Engineer Mountain 7. 4. A small and imperfect ventral valve, which is peculiar by reason of its incurved beak, and its apparently smooth surface without either plications or Specimen seen from Posterior view of Anterior Side view. Ouray limestone; Engineer Mountain 7. Athvris 5. A rather small example of nearly circular shape, in which the two valves have retained their normal Dorsal view. U. S. GEOLOGICAL SURVEY TWENTIETH ANNUAL REPORT. PART II. PL. V. FOSSILS FROM THE OURAY LIMESTONE OF THE UNIVERSITY of ILLINOIS. GIRTY.] DESCRIPTION OF PLATE V CONTINUED. Athyris coloradoensis—Continued. ob. Side view in outline. Tlie convexity of the two valves, as representedin this figure and the one following, seems to show this character asexisting in nearly equal degree in each. Such is far From beinguniversally the case, especially in full-grown shells. The ventralsoften become highly inflated, anil many dorsal valves are but slightlyarched. Ouray limestone; Salida 29. 6. Another undersized shell, better preserved in this regard than the last, in which both valves are retained in their natural position and con-vexity. This specimen was probably the Athyris mentioned In-Meek as occurring associated with Rliynchonella Side view, showing the relative convexity of the two limestone; Station 48. 7. A dorsal valve of transverse type, in
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