214 



ARCADE. 



The shells of this family are easily recognized by the 

 peculiar dentition of their hinges, which are furnished with 

 small comb-like interlocking teeth, all similar, or differing 

 only in size and often very numerous. The characters of 

 sculpture, outline, size, and thickness vary in the several 

 genera. The shells of all, however, are provided with an 

 epidermis. The animals of all the forms have a deeply- 

 grooved foot, capable of expanding into a disk like the foot 

 of a gasteropod. The margins of the mantle are usually 

 freely open, and not formed into tubes posteriorly, though 

 in Lecla^ and probably in Sole?iella, they present the latter 

 arrangement ; consequently, the form of the pallial im- 

 pression in the shell varies in this tribe. There are con- 

 stantly two adductor muscles, the impressions of which are 

 strongly marked on the shells. The structure of the 

 appendages of the mouth afford generic distinctions. 



NUCULA. Lamarck. 



Shell equivalve, inequilateral, shortened anteriorly, ovato- 

 trigonal or obliquely ovate, closed, smooth, or concentrical- 

 ly striated, or (in certain exotic and fossil species) marked 

 by zigzag or radiating furrows ; always invested with a 

 smooth epidermis ; margin denticulated or smooth ; beaks 

 approximated, incurved ; inside nacreous : hinge-line angu- 



