66 MY1D.E. 



the most southern limit in space of the glacial epoch. 

 In the United States it goes by the general name of 

 " clam " ; and Gould informs us that it is more import- 

 ant, in an economical point of view, than the oyster. 

 About 5000 bushels are annually brought to Boston 

 market alone as food for man; and much more than 

 ten times that quantity is salted and used as bait for 

 fish. Its capability of living in brackish and even fresh 

 water is well known. Lindstrom has given the following 

 list of Mollusca associated with it in the Baltic : Neri- 

 tina fluviatilis, Bythinia tentaculata, Physa fontinalis, 

 Limncea st agnails, L. auricularia, L. peregra, Tergipes 

 lacinulatus, Limapontia nigra, Mytilus edulis, Cardium 

 edule, and Tellina balthica. To these may be added 

 several kinds of Crustacea and Hydrozoa. Multitudes 

 of young M. arenaria may be seen in the Loch of S tennis, 

 about 5 miles from Stromness in the Orkneys, attached 

 bv bvssal threads to the under side of loose stones : 

 Neritina fluviatilis lives with them and deposits its 

 spawn on the same stones. Full-grown individuals of 

 the Mya are found (with Littorina obtusata) in the 

 lower part of the loch, which is open to the sea. The 

 fry are squarish-oval, decidedly inequivalve, and not 

 unlike Corbulae. My finest specimen is 3 inches by 5. 

 Lapland seems to produce much larger. 



Gould considers the M. mercenaria and M. acuta of 

 Say synonyms of the present species. 



2. M. trunca'ta*, Linne. 



St. truncata, Linn. S. N. p. 1112 ; P. & H. i. p. 163, pi. x. f. 1-3, and (ani- 

 mal) pi. H. f. 1. 



Body somewhat elongated and compressed, pale brown : 

 tubes very long ; tentacular filaments alternately large and 



* Lopped. 



