35 



ORDER II 



BIVALVES. 



MYA. — Truncate/Trough-shell or Gaper. 



Animal — an Ascidia: Shell bivalve, generally gaping at 



one end; hinge ivith hroad, thick, strong teeth, seldom, 



more tha7i one, and not inserted into the opposite value, 



THIS genus has "been placed by Linnaeus the first on 



the list of Bivalves : its species are by no means numerous, 



forty only having been described by him. 



The principal characteristic of the Mya consists in its 

 gaping at one end : the next general distinguishing mark 

 is, its ha\'ing a single, broad, patulous tooth, proceeding 

 from beneath the beak. This tooth differs from that of 

 the foregoing genus ; for, in the Pholas, it is long and slen- 

 der, and almost of equal size throughout; whereas, in the 

 Mya it is much Avider and broader at one end than the 

 other; and the broadest end has an excavation, which gives 

 it the appearance of the bowl of a spoon or ladle. 



However, this sort of tooth is not always discernible in 

 every species of the Mya, for some are entirely without it ; 

 others, again, have two or three teeth, and, in some in- 

 stances, small crenulations supply the place of a regular 

 hinge. 



The form of the Mya varies exceedingly, some are ob- 



