398 STUDIES IN EVOLUTION 



which takes place after the individual reaches adult size. 

 The thickening may involve the whole interior of the valves, 

 producing a truncate appearance at the margins, or it may 

 take place by frequent interrupted growth along the margins, 

 giving to this portion a varicose aspect. As a result of this 

 senile growth, the vertical diameter of the shell is increased, 

 and the beaks are involuted, so that they are often so closely 

 appressed as to conceal the cardinal area and truncate the 

 ventral beak, and in addition, the margins of the valves lose 

 the characteristic ornamentation of the species and corre- 

 spond to the gerontic stages as defined by Mr. Hyatt.* 



Abnormalities frequently find an explanation in some path- 

 ological or accidental conditions which become instituted at 

 any period in the life of the animal, and leave their impress 

 on the shell. The functional failure of a developing organ 

 may cause the parts to revert to an embryonal type, and 

 although it is difficult to apply this statement to the shelly 

 covering of the animal, yet this condition is sometimes found. 

 The specimen of Oamarotoechia negleeta described on page 342 

 is an instance of this kind. Another abnormal variation is 

 noticed in certain individuals which preserve the larval fea- 

 tures of the shell long after it has passed the early stages, 

 and when, in many cases, it has reached the full adult 

 dimensions. 



* Values in Classification of the Stages of Growth and Decline, with proposi- 

 tions for a New Nomenclature. Proc. Boston Soc. Nat. Hist., XXIII, 1888. 



