374 STUDIES IN EVOLUTION 



Shell convex just below tlie beak, becoming depressed toward 

 the anterior margin. 



Developmental Variations. 



General Form and Outline. — In the incipient stadia of 

 growth the shell is extremely elongate and quite perfectly 

 oval; the beak of the ventral valve is relatively broad, its 

 lateral margins having a slight outward curve. With growth 

 the shell broadens, and the ventral beak becomes more atten- 

 uate, while the greatest width of the shell, instead of being 

 at or below the middle, comes nearer the hinge-line. 



Beak. — From being erect, straight, and relatively broad 

 in the ventral valve, at the outset, it becomes, at maturity, 

 narrow, attenuate, and slightly incurved toward the apex. 



Foramen. — In the earliest observed stage the foramen is 

 a broad, triangular opening, covering nearly the entire car- 

 dinal area, reaching, but not encroaching upon, the apex of 

 the valve. In subsequent stages of development this open- 

 ing narrows with the narrowing of the beak but, as at no 

 stage deltidial plates are developed, the contraction is due to 

 the encroachment of the cardinal portions of the valve along 

 the foraminal margins. The interesting fact of the persistent 

 absence of deltidial plates throughout the entire existence of 

 the individual may be interpreted as a retention to maturity 

 of a character embryonic in allied species ; the small size of 

 the mature shell and the very slight incurvature of the ven- 

 tral beak also contribute to the embryonic expression of the 

 species. 



Whitjieldella nitida Hall, 1843. 

 (Plate XXI, figures 6-10.) 



Meristina nitida Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. 



Hist., p. 160, pi. 25, figs."l-7. 1879. 

 Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 300, pi. 25, 



figs. 1-7, 1882. 



Whitfieldella nitida is a very abundant and characteristic 

 fossil in the Niagara fauna of central Indiana, reaching a 

 much greater development both in size and numbers than in 



