ORBICULOIDEA 39 



Family DISCINID^E 

 Genus ORBICULOIDEA d'Orbigny 



Description. Shell subcircular or subelliptical in outline, inequivalvate. 

 Pedicle valve nearly flat or depressed-convex, with the apex excentric 

 and inclined slightly towards the posterior margin; a pedicle groove 

 originates at the apex of the valve, passes posteriorly for a short distance, 

 beyond which point it is continued as a pedicle tube which pierces the 

 substance of the valve very obliquely, opening to the interior near the 

 posterior margin. Brachial valve depressed conical, with the apex ex- 

 centric and inclined posteriorly. Surface of the valves smooth, or more 

 commonly marked by concentric lines, and more rarely by fine radiating 

 costae. 



Remarks. The species of this genus are most commonly represented 

 by the subconical brachial valves, and it is from the form, proportions, 

 and surface characters of these valves that most of the species have been 

 differentiated. Specimens of the pedicle valves are less commonly pre- 

 served, and when seen the course of the oblique pedicle opening can rarely 

 be traced. 



ORBICULOIDEA CAPAX (White) 

 Plate I, Fig. 13 



1862. Distinct capax White, Proc. Boston Soc. Nat. Hist., vol. 9, p. 30. 

 1900. Orbiculoidea capax Weller, Trans. St. Louis Acad. Sci., vol. 10, 

 p. 65, pi. 1, fig. 19. 



Description. Shell below medium size, subcircular in outline, length 

 and breadth probably about equal. The dimensions of the holotype are: 

 maximum diameter 22 mm., convexity of brachial valve approximately 

 5 mm. 



Pedicle valve not known. 



Brachial valve depressed conical, the apex excentric posteriorly and 

 inclined towards the posterior margin, apparently situated about one- 

 fourth or one-fifth the length of the valve from the posterior margin, the 

 greatest convexity of the valve in front of the apex; the surface gently 

 convex from the apex to the anterior margin, sloping more abruptly to 

 the lateral margins, and a little concave to the posterior margin. The 

 surface marked by fine, regular, concentric lines of growth, and by more 

 or less obscure radiating lines. 



Remarks. The holotype of this species, which is the only example seen, 

 is a much crushed and imperfect brachial valve, the entire outline of 

 which cannot be determined. The central portion of the valve, however, 



