386 PROCEEDINGS OF THE NATIONAL 3IUSEUM. vol. 56. 



by two elevations which become plications and between them is 

 soon developed a single costa which immediately bifurcates. The 

 four plications increase in strength and become strongly elevated 

 as they proceed to the anterior margin producing the conspicuous 

 fold of this valve." * Mature specimens which have well preserved 

 beaks show the presence of a distinct fold on the pedicle valve and a 

 distinct sinus on the brachial valve in early nepionic development as 

 was first pointed out by Professor Cumings. 



During the early neanic stage the shell takes on group character- 

 istics. These groups will be described later as the Uniplicate, 

 Biplicate, and Triplicate Groups. 



In late neanic and early ephebic development the changes which 

 take place vary with different species and are of specific, varietal, 

 or individual value. 



Adult specimens show the hinge-line to be long and straight. Gen- 

 erally it about equals the width across the middle. A few species have 

 the hinge-line much less and a few others much greater than the width 

 farther forward. The cardinal areas are common to both valves and 

 are about equally developed. That of the pedicle valve is generally 

 somewhat the broader. Each valve has an open delthyrium, of which 

 the one in the pedicle valve is the larger. 



The teeth are thick and prominent. In the brachial valve a cardi- 

 nal process is sometimes present as a short, slender, vertical ridge in 

 the roof of the open delthyrium. The dental sockets are small and 

 the crural plate large and strong. The muscular scars are usually 

 excavated into the shell substance. They are small and are not easily 

 separable into their respective parts. An exception is found in 

 Platystrophia ponderosa, in which the scars are separated by a septum 

 in the brachial valve, extending toward the front as a low ridge and 

 dying out before reaching the front. 



About one-third of the distance from the beak the septum sends off 

 ridges at right angles to the general direction of the septum (pi. 52, 



fig. 10). 



Both valves are convex. They vary from flatly to strongly con- 

 vex. The brachial valve may about equal the convexity of the pedicle 

 valve, but it is generally greater. This valve carries a median fold, 

 which may be broad, round, or flat, and but slightly raised above the 

 general convexity of the valve; it may be broad, round, and much 

 elevated, or it may be compressed and greatly elevated above the 

 general convexity. 



The pedicle valve has a median sinus, which may be broad and 

 shallow, broad and deep, or narrow and deep. In some species the 

 depth of the sinus is in proportion to the height of the fold, but some 



> Geol. and Nat. Hist. Surv. Minnesota, vol. 3, pt. 1, 1893, p. 456. 



