160 



III. MYAD^. 



THE GAPER TRIBE. 



To arrange Mollusks, or the genera of any other class of 

 animals, in a sequence of their natnral affinities, is not pos- 

 sible in a written treatise, and can be clone only by means of 

 diagrams. The tribe of bivalve shell-fish which has now 

 to engage our attention is an instance in point, for in many 

 respects the animal of a Mya is much more nearly allied to 

 an Ascidia than are most of the genera of the two tribes 

 we have just passed in review. Yet to introduce the 

 Myad(z between the PlioladidcE and Gastrochamida^ would 

 be to separate, by an unnatural break, most natural 

 alliances. The family before us, if indicated in a diagram, 

 would rather take rank alongside the Pholas tribe, and like 

 it be seen conducting us by gradual transitions from the 

 Tunicata towards the more typical Lamelli hrancJiiata. 

 The aspect of a Mya, when clothed with its coriaceous epi- 

 dermis is that of an elongated Cynthia, against whose sides 

 two plates of shell have been appressed, and no better 

 mode could be devised of impressing on the tyro in mala- 

 cology the close affinity of two great sections, so unlike in 

 most of their proper members, than the placing before him, 

 side by side, examples of the two genera just mentioned. 



The popular appellations of "Gapers" may be applied to 

 the whole tribe. The shells are oblong and somewhat rude 

 in appearance, always more or less gaping, and often very 



