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; foi-amen 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 ram. 



Variations from the Normal Adult. — Two plications some- 

 times occur in the sinus, and in such cases thej^ 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 b). — 

 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 



