328 STUDIES IN EVOLUTION 



valve uniformly increases in size from the youngest forms to 

 full-grown specimens. 



Specific Characters. 



Mature Form (Plate XVI, figures 15, 15 a, 17, 20). — Shell 

 semi-circular or semi-elliptical, depressed convex; hinge-line 

 longer than the width of the shell ; cardinal angles flat and 

 extended. 



Dorsal valve moderately and uniformly convex except at 

 the cardinal angles; umbo not defined; beak small. 



Ventral valve convex on the umbo, less convex below, and 

 in many specimens the marginal portion is flat or slightly 

 concave; beak small, somewhat arched. Hinge-area nearly 

 equal in both valves, usually appearing as a deep angular 

 groove along the cardinal margin. Under the beak of the 

 dorsal valve is a large triangular callosity, grooved on the 

 inside, and nearly filling the fissure of the opposite valve. 

 Deltidium of the ventral valve broadly triangular, extend- 

 ing to just below the beak, and margined on each side by 

 two narrow areas in the form of scalene triangles, which 

 may represent the deltidial plates of other genera. Beak 

 imperforate. 



Surface marked by from fifty to one hundred (according 

 to the size of the shell) regular, rounded strife, with equal 

 interspaces, increasing in number by interstitial additions. 

 The entire shell is also ornamented with very fine, regular, 

 sharp, concentric striae. A large specimen has a length of 

 26 mm., and the width, measured along the hinge-line, is 

 about 38 mm. 



Incipient Form (Plate XVI, figures 14, 14 a). — The small- 

 est specimen measures 1.5 mm. in length by 2.3 mm. in 

 width along the hinge-line. The outline is semi-elliptical, 

 with the cardinal angles slightly extended. Dorsal valve 

 concave in the upper part, and slightly convex below. Ven- 

 tral valve convex; beak prominent, projecting beyond the 

 hinge-line. 



The hinge characters are not well preserved in this indi- 



