MARINE MOLLUSCS 



205 



The family Myacidce may be recognised by the thick, strong, 

 opaque shells, usually gaping at the posterior end ; the wrinkled 

 epidermis which covers the whole or part of the shell ; and the 

 united siphons, which are more or less retractile. The mantle 

 cavity is also closed with the exception of a small hole left for the 

 protrusion of the small foot. The pallial line of the shell is 

 sinuated. 



FIG. 138. 1. Mya truncata. 2. INTERIOB OF SHELL. 3. Mya 

 arenaria. 4. Corbula nucleus 



In the above illustration we represent the Common Gaper (Mya 

 arenaria), which burrows to a considerable depth in the sand or mud, 

 especially in the estuaries of rivers, from between the tide-marks 

 to a depth of twenty fathoms or more. It may be readily distin- 

 guished, in common with the other species of the same genus, by the 

 characteristic wrinkled, membranous tube that encloses its fringed 

 siphons, the membrane being a continuation of the epidermis that 

 extends over the shell. Another characteristic feature of the genus 



