522 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



ORTHOTHETES Fischer de Waldheim, 1830. 

 Orthothetes in.equalis Hall. 



PL LXVIII, fig. 3a. 



Orthis ineqxialis Hall, 1858: Geol. Surv. Iowa, Vol. I, Pt. II, p. 490, PL II, figs. 6a-6c. 

 Streptorhynchus ineequaUs A. Wmchell, 1865: Proc. Acad. Nat. Sci. Phila., p. 117. 

 Streptoryhnchus equivalnis Hall and Whitfield, 1877: King's U. S. Geol. Expl. 40th 



Par., Vol. IV, p. 252, PL IV, figs. 1, 2. 

 Streptorhynchus cvquivalvis Hall, 1883: Second Ami. Eept. New York State Geologist, 



PL 12, figs. 20-23. 

 Orthothetes incequalis Hall and Clarke, 1892: Pal. New York, Vol. VIII, Pt. 1, PL 9a, 



figs. 20-23. 



Orthothetes incequalis is extremely abundant in the limestones and cal- 

 careous sandy shales of the Yellowstone National Park. The largest shells 

 measure 25 mm. in length by 38 mm. in breadth, but the average is 

 somewhat smaller than this. The shape is semicircular. The outline is 

 somewhat contracted at the cardinal extremities, and the hinge line slightly 

 shorter than the greatest width of the shell. The surface is marked by 

 numerous elevated, sharp, radiating striae, about fourteen in the space of 

 5 mm., which leave between them intervals wider than the stria; them- 

 selves, from which they abruptly rise. They do not bifurcate, but in the 

 widening intervals which result from their radiating direction neAV striae are 

 from time to time introduced. They sometimes become much crowded 

 through the center of the shell and around the periphery. The stria; are 

 not all of the same size. Sometimes they are alternately large and small; 

 sometimes every fourth one is large, but more often there is no conspicuous 

 arrangement, They are crossed by numerous fine concentric stria; charac- 

 teristic of the genus. As a result of the surface structure just mentioned, 

 casts of the exterior are misleading in that they seem to present a form 

 with broad, close-set, and bifurcating radiating plications. 



The dorsal A'alve is usually gibbous, but sometimes is more gently 

 curved. As a rule the curvature is regular from beak to frontal margin, with 

 a somewhat prominent umbo; but forms occur where the umbo is flattened 

 and there is a point of prominent elevation in the middle of the valve; or, 

 where the shell is depressed, with almost a geniculation near the margin. 



