378 STUDIES IN EVOLUTION 



Meristina rectirostris. To the erect, attenuate, acute beak, 

 open pedicle-aperture, shallow valves, and asinuate anterior 

 margin of the latter, the full, rounded, incurved beak, con- 

 cealed cardinal area, ventricose valves, and strongly sinuate 

 anterior margin of M. Maria are strongly contrasted. Be- 

 tween the mature characters of these two species, Whitfieldella 

 nitida is conspicuously mediate. 



Immature specimens of M. Maria are far from abundant. 

 Indeed, the present series shows only about thirteen different 

 grades of development, and the smallest individual which 

 can be referred with certainty to the species measures 6x6 

 mm. (adult 29 mm. in length by 32 mm. in width). 



The writers have, however, assigned to the species a minute 

 embryo measuring .75 X .75 mm., and if this is correctly done, 

 the embryos of this species in the earliest stages of growth 

 differ from those of the other non-plicate species here dis- 

 cussed, in a much stronger tendency toward a circular outline. 



The beak of the ventral valve becomes incurved, and the 

 cardinal area obscured very early, so that the discussion of 

 the development of these parts is necessarily much curtailed. 



Specific Characters. 



Mature Form (Plate XXI, figures 3, 3 a). — Shell compara- 

 tively large, ventricose, broadly ovate or sub-pentagonal. 



Ventral valve gibbous in the umbonal region, with a low, 

 broad dorsum extending from the umbo to near the middle 

 of the valve, where it becomes flattened, sinuate, and at the 

 anterior margin is reflected dorsally into a linguiform exten- 

 sion. Beak closely incurved over the dorsal valve, fully 

 concealing the foramen. Cardinal slopes angulate and 

 slightly excavate. 



Dorsal valve evenly convex, somewhat gibbous, strongly 

 arcuate transversely along the dorsum, which becomes ele- 

 vated into a low fold, deeply emarginate in front for the 

 reception of the extension from the opposite valve. Beak 

 obtuse, incurved, and concealed. 



Surface smooth, marked by concentric growth-lines near 

 the margin. 



