342 STUDIES IN EVOLUTION 



from the normal adult form, and these observations are there- 

 fore limited to an essentially unvarying phase. 



Specific Characters. 



Mature Form (Plate XVIII, figures 8, 8 a). — Shell small, 

 transversely sub-ovate; umbo scarcely prominent. Cardinal 

 slopes long and flattened, rounding to the anterior margin 

 which is nearly straight. 



Ventral valve with the umbo elevated and slightly incurved 

 at the tip, overhanging an elongate sub-triangular foramen. 

 Umbonal region slightly convex, the convexity extending for 

 one -third the length of the shell; thence forward the shell is 

 rapidly depressed medially to form a deep sinus, which makes 

 a high quadrangular extension on the margin ; lateral portions 

 depressed. 



Dorsal valve with the umbo low and inconspicuous; apex 

 concealed within the foramen of the opposite valve. The 

 shell becomes rapidly elevated medially to form the fold, the 

 latera being full and convex. 



Surface covered with regular, sharp, and prominent plica- 

 tions, which do not vary in number at normal maturity, and 

 which, in the growth of the shell, are increased only from the 

 cardinal margins. Of these plications, the fold bears four, 

 the sinus, therefore, three, and each of the latera five, those 

 nearest the cardinal margins being obscure. This makes in 

 all for the ventral valve, thirteen, and for the dorsal, four- 

 teen plications. Dimensions of an average example, length, 

 width, and depth, 9, 8, and 5 mm. 



A single individual presents the only important abnormal- 

 ity noticed ; namely, a failure to produce the requisite plica- 

 tions upon the latera, the dorsal valve bearing but five, and the 

 ventral, six. Of these, three are on the fold, two in the 

 sinus. It is interesting to notice that in the umbonal region 

 the normal number of plications had been formed in their 

 regular arrangement; their disappearance on the latera and 

 irregular disposition in fold and sinus took place abruptly 

 upon the completion of a growth-line 2 mm. from the apex. 



