DELTHYRIS 303 



height, only slightly concave, the lateral margins angular and well- 

 defined ; the delthyrium narrowly triangular, higher than wide ; each lat- 

 eral slope bearing three or four simple, rounded plications which origin- 

 ate near the cardinal margin. Internally the valve bears a distinct median 

 septum. 



Brachial valve less convex than the pedicle/ the greatest convexity near 

 the middle, the surface a little compressed towards the cardinal extremi- 

 ties ; mesial fold rounded, but little elevated above the general surface of 

 the valve ; the plications similar to those of the opposite valve and alter- 

 nate with them. 



Besides the plications the surface of each valve is ornamented by strong, 

 crowded, sublamellose, concentric markings, three or four of which occupy 

 the space of one millimeter. 



Remarks, This species most closely resembles D. clarksvillensis, agreeing 

 with that species in its rather shallow, rounded sinus and low fold, in 

 the rather coarse rounded plications, and in the stronger and coarser 

 concentric lamellose markings of the shell than is usual in other species. 

 D. missouriensis differs from D. clarksvillensis in its somewhat narrower 

 shell and more convex pedicle valve, and in the smaller number of plica- 

 tions. The specimens whose dimensions have been given are among the 

 largest ones observed, a great number of individuals having a width of 

 less than 5 mm. 



Formation. Glen Park limestone and Hamburg oolite of the Kinder- 

 hook. 



DELTHYRIS SUBORBICULARIS Weller 

 Plate XXXVI, Fig. 8 



1906. Delthyris suborbicularis Weller, Trans. St. Louis Acad. Sci., vol. 16 

 p. 446, pi. 6, figs. 27-28. 



Description. Shell small, broader than long, the hinge-line shorter than 

 the greatest width, the cardinal extremities rounded. The dimensions 

 of a pedicle valve are : length 10.5 mm., width 11.7 mm., length of hinge- 

 line 8.7 mm., convexity 3 mm., height of cardinal area 2.3 mm. 



Pedicle valve moderately convex, the convexity extending out to the 

 cardinal extremities, the surface sloping with a gently convex curvature 

 from the umbo to the anterior and lateral margins ; mesial sinus originat- 

 ing at the beak, rather broad and shallow, rounded in the bottom; the 

 beak obtuse, scarcely incurved ; cardinal area slightly concave, sloping 

 posteriorly from the hinge-line and lying at an angle of about 130 degrees 

 to the plane of the valve, the lateral margins not sharply defined, the 

 surface rounding into that of the lateral slopes of the valve ; delthyrium 

 higher than wide; each lateral slope bearing five or six simple, low, 

 rounded plications which originate near the cardinal margin and grow 



