348 STUDIES IN EVOLUTION 



Variations from the Normal. — (A) Forms with one plication 

 in the central sinus. This variation does not attain quite the 

 size of the average normal adult, but retains the same pro- 

 portion of length and breadth (size 10 x 10 mm.). The 

 surface bears ten plications on the dorsal and nine on the 

 ventral valve. In this form the embryonal sinus, visible on 

 the earlier portion of the dorsal valve, is distinctly continu- 

 ous with the strong sulcus separating the two plications on 

 the fold in the later and marginal portions of the valve. 

 This variation is not of uncommon occurrence, and immature 

 individuals in various stages of development prove that it is 

 a well-established genetic difference, and not merely an occa- 

 sional monstrosity. 



(B) Forms with three plications in the ventral sinifs. The 

 size and proportions of the normal are retained in this variety, 

 but the shell bears usually three, sometimes four plications 

 on each of the latera, making ten (twelve) plications for the 

 dorsal, and nine (eleven) for the ventral valve. This form is 

 of comparatively rare occurrence, and is not often noticed in 

 an immature stage of growth. 



(C) Forms ivith four plications in the ventral sinus. This 

 variation is met with very infrequently, but two individuals 

 having been obtained. While agreeing in size with the nor- 

 mal adult, the crowding of the sinus with plications tends to 

 obliterate both it and the fold upon the opposite valve. 

 Both individuals show the interesting fact that upon the 

 dorsal valve where the fold bears five plications, that is, 

 four sulci, the embryonal sinus is continuous with the third of 

 these sulci, in one instance numberino: from the rio^ht, in the 

 other numbering from the left. Of the five plications which 

 are thus separated into groups of three and two, it is notice- 

 able that the outer member of the group of three is both less 

 elevated and shorter than any other upon the fold. 



Monstrous Forms. — The sole evidence of monstrous growth 

 observed is an asymmetrical development of the plications 

 upon the dorsal fold. Examples bearing three plications upon 

 the fold, in rare instances have one of the plications very 



