34 MEMOIRS OF THE STATE MUSEUM 



In the smallest specimen, the depth of both valves is less than one-half 

 the length of the shell. This relation gradually changes as the shell 

 becomes larger and more convex, until, in mature individuals, the depth is 

 equal to three-fourths or four-fifths the length of the shell, and in extremely 

 obese specimens this ratio is often exceeded. 



The dorsal valve is more convex than the opposite valve, in all the 

 stages of growth which have been observed, although in the elemental shell, 

 the difference is scarcely perceptible^ while in the mature form it is a con- 

 spicuous feature. This valve is also shorter than the ventral in specimens 

 up to a length of about 7 mm. From 7 to 12 mm., both valves are of nearly 

 equal length. Further growth causes the umbo of the dorsal valve to pro- 

 trude beyond the beak of the opposite valve, and the beak is incurved and 

 penetrates the area. It seems evident that if we had the true initial shell, 

 the dorsal valve would be found not only smaller but less convex than the 

 opposite valve. 



The fold begins to be apparent in individuals having a length of about 

 10 mm., and is expressed by the arching of the anterior margin. It does 

 not sufficiently develop to become a characteristic feature and is more or 

 less undefined, even in many full-grown specimens. 



The plications increase both by bifurcation and interstitial addition. 

 The smallest number observed is eight, and this is gradually increased with 

 the growth of the shell, until there are about fifteen principal plications on 

 the body of the shell, and several smaller ones just below the cardinal 

 extremities. The concentric striae are not often preserved and the plica- 

 tions therefore form the only conspicuous character of the surface orna- 

 mentation. 



The deltidial plates reach but a slight development, the triangular 

 opening becoming completely filled by the incurved beak of the dorsal 

 valve. 



