The Nautilus. 



Vol,. XVIII. NOVEMBER, 1904. No. 7. 



PANOPEA BITRUNCATA CONRAD. 



BY CHAItLKS ^V. .JOHNSON. 



The sliell of the genus Panopea, like most of the burrowing 

 Pelecypods is subject throughout its growth to considerable variable- 

 ness. The causes of mutation are so admirably described by Dr. 

 Dall ' that I quote in full the following paragraphs: 



" All boring moUusks in which the shell has so degenerated that 

 it no longer covers the whole adult animal when retracted are more 

 liable to variation in minor details than those in which the valves 

 meet distally, and dynamically influence their own development by 

 fixing for it certain definite limits. This is markedly the case in the 

 present genus. Those shells which live in an easily movable 

 medium, such as sand or tine, soft mud, are thinner, better devel- 

 oped, more elongated and less distorted than their congeners who 

 are obliged to confine tliemselves to a gravelly or stony situs. So 

 marked is the difference that 1 have several times been presented 

 with supposed new species based on these dynamic character?, and 

 by a curious reversal of logic, have been assured that the differences 

 must be specific, because the animals iniiiibifed, respectively, the 

 different kinds of ground alluded to. 



" I have observed, also, tiiat where the ground into which the 



1 Contributions to the Tertiary Fauna of Florida, by William Healey Dall. 

 Transactions of the Wagner F^ree Institution of Science, Vol. Ill, part IV, page 

 827. Philadelphia, April, 1898. 



