254 



A MONSTROSITY OF MACTRA STULTORUM Linne. 



By Dr. W. G. N. VAN DER SLEEN. 



(Read before the Society, February gth, 1916). 



In the course of the last three years I have frequently come across 

 specimens of Madra stultorum L. whose shell showed a very 

 abnormal form. When finding an empty shell of this" form the first 

 time, I instantly compared it with Mya truncata, but this comparison 

 is false in so far as the opening in the Mactra shell was not in the 

 place of the sipho, but allowed the foot to be protruded without 

 opening the shell. 



Another stroll along our sandy beach brought me half-a-dozen 

 specimens of the sought form and all of them still contained the 

 animal. As far as I could find out, the animal was quite normal ; 

 only in one individual I found a small specimen of Pinnotheres pistim, 

 which was, according to the literature in my possession, until now not 

 found in Madra stultorum. As the reason of the abnormality seems 

 not to lie in the animal, I have tried to find it in its surroundings. 

 When a young specimen of Madra stultorum lives near our sandy 

 coast, it will have hard work not to be buried so far under the sand, 

 that this might be its cause of death. On the other hand, when 

 living in the top of a sand-bank, it will want all the strength of its 

 foot not to be swept off by the waves or tides. In a few words, the 

 normal attitude of the animal will be to have the foot quite extended, 

 and when now the growing of the shell goes on, it will form a shell 

 with a large opening for the protruded foot. Specimens, however, 

 that live in quiet, deeper water not so near the coast will generally 

 have the foot quite inside the shell, and so form shells that are 

 quite closed. 



If in the period of shell-forming the animal is first in the range of 

 the quickly-moving sand, and afterwards comes into deeper, more 

 quiet regions, where the foot need not be always protruded, the shell 

 generally gets closed again in short time, a bubble naturally staying 

 on each valve. 



