320 STUDIES IN EVOLUTION 



Developmental Changes. 



General Form and Outline. — In the growth of the shell a 

 change becomes manifest in its outline and relative propor- 

 tions. The young stages have the width greater than the 

 length, but the more rapid axial growth of the shell reverses 

 these proportions at maturity. Moreover, in the incipient 

 stages, the valves, as already noticed, are of nearly equal 

 depth and convexity. In the next stage the depth of the 

 ventral valve has noticeably increased over that of the dorsal, 

 and, as in the latter valve the median sinus has become 

 distinctly developed, the difference in this respect becomes 

 emphasized. The divergence of the valves in convexity be- 

 comes increased until maturity, and this growth is accom- 

 panied in the ventral valve by a correspondingly increasing 

 incurvature of the beak. 



Beaks. — In the incipient shell the beaks are erect and 

 distant, but not prominent. By the development of the 

 broad sinus on the dorsal valve, the beak of this valve be- 

 comes relatively less prominent and apparently more closely 

 appressed to the cardinal line. On the opposite valve every 

 increase in convexity is accompanied by a corresponding 

 increase in the incurvature of the beak; and as the shell 

 approaches maturity, the incurvature becomes so great that 

 it has been necessary, in the drawings which are here given 

 showing the features of the cardinal area, to represent the 

 beak as broken away. 



Foramen. — The earliest stages of growth show a remark- 

 able feature in the triangular, marginate, sub-equal fissures 

 on the valves. This character may prove of a high taxo- 

 nomic value. The foramen upon the ventral valve is, in 

 every stage of development, open and free for the pro- 

 trusion of the pedicle. Deltidial plates are absent in every 

 stage of growth. In a secondary stage a cardinal process 

 begins to form in the apex of the dorsal foramen, soon widen- 

 ing and becoming tripartite. As age increases, this process 

 is projected into the ventral foramen, never quite filling it, 



