370 STUDIES IN EVOLUTION 



Specific Characters. 



Mature Form (Plate XIX, figures 19, 19 a, 195). Shell 

 small, sub-oval or cordate, often sub-pentagonal; plano- 

 convex, greatest width along the hinge. 



Ventral valve convex, depressed at the sides ; beak exsert, 

 and in old forms arcuate. 



Dorsal valve flat and sometimes depressed near the beak. 

 The ventral valve is marked by two very prominent plica- 

 tions which pass along the deep median sinus, and are accom- 

 panied by two less distinct plications on each of the latera, 

 making in all six plications, of which the two nearest the 

 hinge -line are sometimes obsolescent. The dorsal valve bears 

 a prominent median fold, and two well-marked plications on 

 the lateral portions of the shell. Toward the margin are 

 concentric laminae of growth. An average adult individual 

 measures 5.5 mm. in length by 5 mm. in width. 



Variations in Outline. The individuals divide themselves 

 into two groups according to their outline : 



(a) Normal form. 



(b) Long form. 



The first of these groups includes the great majority of all 

 individuals found, which are characterized by a relatively 

 broad figure and sub-circular outline. Members of the 

 second group are comparatively few in number, and are 

 elongate or sub-ovate individuals. The long form (b) is well 

 defined in immature growth-stages, and appears to be a 

 permanent varietal difference. 



Abnormalities. A variation in adult shells, noticed only 

 in rare instances, is a tendency to an asymmetrical develop- 

 ment in the plications, as shown on Plate XIX, figure 18, where, 

 by unequal growth upon the lateral portions of the shell, the 

 median plication on the dorsal valve is deflected to one side, 

 and the corresponding median sinus on the ventral valve dis- 

 placed, the axial line of the shell being occupied by one of 

 the strong plications bounding the sinus. Another form of 

 this asymmetry is manifested in the intercalary addition of a 



