340 STUDIES IN EVOLUTION 



fuse it with any of the associated species arises only among 

 forms of immature growth. Beginning with a shell which is 

 apparently in the actual initial stage, measuring 1.2 x .8 mm., 

 the present series is very evenly consecutive up to maturity, 

 when the average dimensions are 8x6 mm. 



Specific Characters. 



Mature Form (Plate XVIII, figures 11-11 h). — Shell small, 

 longitudinally ovate, sub-attenuate toward the beak, and 

 truncate in front. Cardinal margins long and rapidly slop- 

 ing, extending more than half-way across the shell; sides 

 flattened, slightly excavate. Valves sub-equally convex. 



Ventral valve full and rotund on the umbonal region, flat- 

 tened at about the middle, thenceforward sinuate ; beak in- 

 curved, but not procumbent; foramen generally concealed, 

 or when slightly exposed, elongate or sub-triangular. 



Dorsal valve more flattened in the umbonal region and in 

 the middle, whence a low fold proceeds to the margin. 



Surface marked by low rounded plications. The ventral 

 sinus bears a single plication which is generally faint, often 

 nearly obsolete. On each side of the sinus are four plica- 

 tions, those abutting on the cardinal margins being indis- 

 tinct. On the dorsal valve the low, flattened fold bears two 

 plications which are the strongest upon the shell; these are 

 accompanied by three plications on each latus, making the 

 whole number on this valve eight. No concentric growth- 

 lines are apparent. Average dimensions 8x6 mm. 



Variations from the Normal Adult. — Two plications some- 

 times occur in the sinus, and in such cases they are each 

 stronger than the single sinal plication in the normal adult. 

 The addition of the plication to the sinus increases the num- 

 ber in the fold to three, and the total number of plications on 

 the shell by two. 



Initial Shell (compare Plate XVIII, figures 9, 9 a, 9 5). — 

 Two individuals, one measuring 1.2x8 mm., the other 

 1.4 x 9 mm., apparently indicate the initial stages in the 

 growth of this shell. Neither of these examples has served 

 well for illustration, on account of the lack of well-defined 



