362 STUDIES IN EVOLUTION 



plications. A marked illustration of this is afforded by an 

 individual which, in repairing an injury to its shell, has 

 abruptly developed six plications on one side of the sinus, 

 in continuation of three and to correspond with three on 

 the opposite side. 



The absence of plications in the sinus. This is a feature 

 of rare occurrence, and is undoubtedly an infantile character 

 retained in later stages of growth. A single individual of 

 immature growth affords an illustration of a peculiar abnor- 

 mality, indicating a reversal in the growth to an embryonic 

 condition. This shell (Plate XIX, figures 3, 3 a, ) has grown 

 to a certain size and normally developed its plications, but an 

 abrupt period has been placed to their development, and over 

 the entire anterior portion of the individual, in front of a 

 stout varix, the surface of the shell is almost smooth. This 

 is the exact counterpart of that mode of growth to which 

 attention is called under other species, where the smooth 

 embryonic condition of the shell seems to be prolonged for 

 more than the usual period of immaturity, and the mature 

 features are thereupon abruptly developed after the formation 

 of a sharp growth-line. 



Developmental Variations. 



The series of individuals illustrating the embryological 

 changes in this species is so complete as to show by almost 

 imperceptible gradations the entire chain of development 

 from very near the starting-point up to maturity. This 

 series begins with an individual measuring 1 mm. in length 

 and .8 mm. in width, and at this stage of growth the incipi- 

 ent shell has manifestly not received much increment. That 

 this, however, is not the actual primitive shell seems proved 

 by indications of two very indistinct concentric growth-lines, 

 and by the presence of faint radiating plications near the 

 anterior margin, between the second growth-line and the 

 margin itself. It is very probable that the incipient shell 

 consisted of that portion of the individual (Plate XIX, figure 

 1) lying within the first growth-line, and as this would make 



