SCHIZOPHuRIA 163 



although the cotypes of the species include specimens from the superjacent 

 bed at the same locality. These latter specimens, however, grow to a 

 much larger size, besides differing in some other respects, and should 

 possibly be referred to the species here described as 8. chouteauensis. In 

 some respects 8. subelliptica suggests a diminutive 8. swallovi, but the 

 surface of the pedicle valve is much less flat than that species, with the 

 beak and umbo proportionally more prominent. The radiating costoe are 

 also less nearly uniform in size in this species, with the external openings 

 of the radiating canals restricted to the larger costa3. The brachial valve 

 has not been available for study, but it is believed that it much more 

 nearly equals the pedicle valve in convexity than in 8. swallovi, in which 

 species the convexity of the brachial valve greatly exceeds that of the 

 pedicle. 

 Horizon. Kinderhook. 



SCHIZOPHORIA CHOUTEAUENSIS n. sp. 



Plate XXIII, Figs. 6-19 



Description. Shell of medium size or smaller, broader than long, trans- 

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

 of the shell, the cardinal extremities rounded. The dimensions of two 

 nearly perfect individuals are : length of pedicle valve 19.9 mm. and 

 12 mm., length of brachial valve 19 mm. and 11.8 mm., greatest width 

 22.8 mm. and 14.2 mm., length of hinge-line 14.4 mm. and 10 mm., thick- 

 ness 11.2 mm. and 7 mm. 



Pedicle valve most convex near the umbo, the surface curving rather 

 abruptly to the cardinal margin and more gently to the lateral and 

 anterior margins, a little compressed towards the cardinal extremities ; 

 mesial sinus entirely obsolete ; umbo prominent with the beak rather 

 small and a little incurved; the cardinal area small, its lateral margins 

 well defined, concave with the curvature increasing towards the beak, the 

 lower and flatter portion lying at nearly a right angle to the plane of the 

 valve. Internally the cardinal teeth are supported by short dental lamellae 

 which continue anteriorly along the inner surface of the valve as raised 

 ridges bounding the muscular scars laterally; the muscle scars fall short 

 of reaching the middle of the valves anteriorly, they are subcordate in 

 outline with a deep, acutely angular emargination in front, and are 

 divided longitudinally by a raised, subangular ridge, which is highest, 

 anteriorly at the base of the emargination and is rather rapidly reduced 

 in height posteriorly. 



Brachial valve a little more convex than the pedicle, the greatest con- 

 vexity posterior to the middle, the surface curving abruptly to the car- 

 dinal margin and more gently to the lateral and anterior margins, com- 

 pressed towards the cardinal extremities; the mesial portion of the valve 



