338 STUDIES IN EVOLUTION 



Dorsal valve gibbous, with the central portion elevated, 

 frequently presenting a broad undefined median fold; beak 

 incurved under the beak of the opposite valve; umbo 

 prominent. 



Surface marked by about fifteen strong, simple, elevated, 

 rounded or angular plications on the body of the shell, and 

 smaller bifurcating plications on the latera. Occasionally 

 intercalated plications are present on the middle of the 

 valves. The plications are crossed by fine arching stria of 

 growth, which are sometimes aggregated, forming conspicuous 

 concentric lines or varices of growth. 



Mature shells measure from 11 to 17 mm. in length, and 

 from 12 to 19 mm. in width. The depth of the conjoined 

 valves varies from 9 to 12 mm. 



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

 est shell observed has a length of 2 mm. and a width of 

 2.25 mm. The dorsal valve is slightly more convex than 

 the ventral, and is a little shorter. Eight rounded plications 

 are shown, five of which extend to the umbo of the valve. 

 A short plication is intercalated in the middle, and there is 

 also a short one on each side of the valve. Ventral beak small 

 and elevated, with a broad, triangular, open area below. 



Developmental Changes. 



The series of specimens selected to represent the develop- 

 ment of this species contains fifteen normal individuals, vary- 

 ing from a length of 2 mm. to a length of 17 mm. The 

 proportions of length and width remain nearly constant 

 throughout, the width being somewhat the greater. 



In the smallest specimen the depth of both valves is less 

 than one-half the length of the shell. This relation grad- 

 ually 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, 



