324 STUDIES IN EVOLUTION 



middle, and about one-fourth greater than the length. Dorsal 

 valve convex in the upper part, becoming concave toward the 

 front. The hinge-area of this valve is very narrow and 

 linear, and carries beneath the beak a small grooved callosity. 

 Ventral valve convex, sloping in all directions from near 

 the foramen, around which the surface is slightly depressed. 

 There is also a depression extending along the middle of the 

 valve to the anterior margin. The place of the beak is occu- 

 pied by an exsert, conical pedicle-tube, which partly pro- 

 trudes beyond the cardinal margin of the valve and extends 

 down to, and embraces the dorsal callosity. Cardinal area 

 of the ventral valve comparatively broad, narrowing rapidly 

 from the pedicle-tube to the extremities. Surface smooth, 

 except along a narrow zone around the margin, which shows 

 incipient radiating striae. 



Developmental Changes. 



The form of this species being somewhat complex, the 

 development of the shell may be conveniently sub-divided 

 into four stages, briefly characterized as follows : — 



\st Stage. Length of shell .4-1 mm. ; surface smooth. 



2d Stage. Length 1-2 mm. ; shell radiatingly striated, with- 

 out undulations. 



Zd Stage. Length 2-20 mm. ; shell radiatingly striated, and 



concentrically undulated. 

 ^th Stage. Length 20-30 mm. ; entire shell radiatingly stri- 

 ated, concentrically undulated in the upper 

 part, abruptly produced and geniculated in 

 front. 



The changes taking place in the form and character of the 

 shell from one stage to another can be best shown and used 

 for comparison in the following tabulation, where the condi- 

 tions incident to each stage of growth in the various parts of 

 the shell are briefly described : — 



