24 MEMOIRS OF THE STATE MUSEUM 



and nearly filling the fissure of the opposite valve. Deltidium of the ven- 

 tral valve broadly triangular, extending to just below the beak, and mar- 

 gined 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 striae, with equal interspaces, increasing in num- 

 ber 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 2, figs. 14, 14^;). The smallest specimen 



measures 1.5 mm. in length, by 2.3 mm. in width along the hinge-line. The 

 outline is semielliptical, with the cardinal angles slightly extended. Dorsal 

 valve concave in the upper part, and slightly convex below. Ventral valve 

 convex ; beak prominent, projecting beyond the hinge-line. 



The hinge characters are not well preserved in this individual. The 

 first specimen in the ascending series which shows the hinge distinctly, has 

 a length of 2.25 mm., and will be described in the development of this part. 



The surface of the incipient shell is marked by seventeen alternating, 

 narrow, elevated radiating lines, with wider interspaces, and also shows sev- 

 eral lines of growth near the margin. 



DEVELOPMENTAL VARIATIONS 



No marked changes occur in the general form of the shell, other than 

 the gradual increase in the convexity of the dorsal valve and in the exten- 

 sion of the cardinal angles. The dorsal valve is usually quite flat in speci- 

 mens having a length of 10 mm. or less. The radiating lines increase in 

 number by interstitial additions, from the youngest form to maturity, and 

 the fine concentric striae appear on all the specimens, including the initial 

 individual in the series, where they are developed around the margins 

 of the valves. 



The earliest phase of the hinge yet noticed, is found in a specimen 



