SCHIZOPHORIA 165 



length of the valve, not emarginate in front, divided longitudinally along 

 the median line by an abruptly elevated angular ridge which is highest 

 near the front and is gradually reduced in height towards the beak, in 

 front of the muscular scar the median ridge is produced anteriorly as 

 a flattened rib upon the inner surface of the valve. 



Brachial valve more convex than the pedicle, the greatest convexity 

 posterior to the middle, the surface curving abruptly to the cardinal 

 margin and more gently to the lateral and anterior margins, compressed 

 towards the cardinal extremities ; the median portion of the valve either 

 not differentiated from the general curvature of the surface or broadly 

 and indefinitely flattened from the umbonal region to the front ; the umbo 

 a little produced beyond the cardinal margin, the beak small and in- 

 curved; the cardinal area small, lying in nearly the plane of the valve. 

 Internally the socket plates are prominent and widely divergent, the 

 cardinal process is small, and is not produced as a median rib along the 

 median line of the inner surface of the valve; the muscular scars are 

 subovate in outline and are about equal in size to those of the opposite 

 valve. 



Surface of both valves marked by fine, radiating, rounded costas which 

 increase by bifurcation and intercalation, and about three of which occupy 

 the space of one millimeter near the front of a mature example. The 

 costae are crossed by a few more or less indistinct, concentric lines of 

 growth. The tubular nature of the costaa has not been observed. 



Remarks. This species most closely approaches 8. swallovi in its char- 

 acteristics, but it differs from that species in the more convex pedicle 

 valve and in the more nearly equal convexity of the two valves. In- 

 ternally the shape of the muscular scars of the pedicle valve differs from 

 those of S. swallovi in being without the emargination in front, and 

 in having the median septum which divides the muscular scar longi- 

 tudinally, produced anteriorly as a depressed ridge. In general contour 

 the species most closely resembles 8. cliouteauensis, but it is larger than 

 that species and has the pedicle valve indefinitely flattened towards the 

 front, besides having the same difference in the form of the muscular 

 scar of the pedicle valve as between it and 8. swallovi. 



Horizon. Upper Chouteau limestone. 



SCHIZOPHORIA POSTSTRIATULA n. sp. 

 Plate XXII, Figs. 7-14 ( ?15-17) 



1909. Schizophoria swallovi Weller, Bull. Geol. Soc. Am., vol. 20, p. 296, 

 pi. 12, figs. 6-7. 



Description. Shell resupinate, above medium size, wider than long, 

 subelliptical in outline, the greatest width near the mid-length of the 



