MYA. 63 



pedal opening ; but although this mode of introduction 

 may take place to a certain extent when the Mya or 

 Pholas is removed from its hole, and placed in a vessel 

 of water (after having ejected the greater part of its 

 fluid contents, so as to create a vacuum), it is difficult 

 to conceive how the requisite supply of food and 

 water can be thus procured while the Mya is imbedded 

 several inches in impervious clay or the Pholas is en- 

 closed in its stony cell, or what in either of the above 

 cases would be the use of the larger tube. I have 

 repeatedly witnessed in many kinds of Bivalve INIollusca 

 a cuiTcnt charged with animalcula or molecules being 

 absorbed by this tube in a continuous stream, and a 

 limpid current discharged at the same time by the 

 smaller tube, occasionally together with pellets of fsecal 

 matter or other rejectamenta. The structure of the 

 shell has been investigated by Dr. Carpenter, and found 

 to consist of variously formed cells : in the tooth or 

 hinge-process is seen a group of large cells, the calcareous 

 contents of which are arranged on a very regularly radi- 

 ating plan, like that of the mineral called Arragonite 

 or Wavellite. Neither in the shell nor in the tooth is 

 there animal matter enough to give anything more than 

 a delicate membranous residuum, in which no vestige 

 of cell- walls can be detected. 



This genus is modern in a geological sense, and does 

 not occur in any formation older than the upper terti- 

 aries. Only three species live in the European seas, 

 the larger two of which are edible. 



