CHAP. X, BIVALVE SHELL-FISH GENERALLY. 253 



close our survey of the TectihrancMa, and with it the 

 whole of the pre-eminently typical Testacea. The 

 little space now left_, will therefore be devoted to the sub- 

 typical order. 



CHAP. X. 



ON THE ORDER DITHYRA, OR BIVALVE SHELLS. THE AFFINITIES 



AND ANALOGIES OF THE UNIONID^, OR RIVER UNIOS. 



(238.) Our survey of the gastropod shell-fish has so 

 much exceeded the limits we originally intended, that 

 we can do little more than bestow a hasty glance en the 

 remaining Testacea. Comparatively so few of the forms 

 among the Gasteropoda had been defined by our pre- 

 decessors, that it was absolutely necessary to treat that 

 order in much detail : but this is not t'le case with the 

 DiTHYRA ; nearly the whole of the genera and sub-genera 

 have been defined; with the single exception of thellNio- 

 NiD^, to the illustration of which we have consequently 

 devoted the greatest part of this chapter. A few remarks 

 on the remaining families, with an exposition of their 

 analogies, is all that our space will admit. We could, 

 indeed, have easily made this single order alone the sub- 

 ject of another volume; but as nearly all the genera may 

 be said to be established, and their technical distinctions 

 are given in the systematic arrangement, the reader must 

 be satisfied with the results rather than with the detaih 

 of our analysis. 



(239.) The DiTHYRA *, or bivalve shell-fish, are ani- 

 mals whose general structure resembles that of the oyster. 

 They are without any distinct head, and are inclosed in 

 two shelly valves, which, by means of certain muscles 



* Comprising the first order of Cuvier's Acephala, the seventh order of 

 his Gasteropoda, and the fifth class of his MoUusca. 



