CHAP. X. UNIONIDJS, GENERALLY. 259 



types always followed each other^ and that they consti- 

 tuted the aberrant divisions. On looking to the typical 

 and the sub-typical divisions of the larger groups, we 

 find that the first are the most perfectly closed bivalves, 

 or the most highly finished and ornamented ; while the 

 latter seem not to have so much the power of free 

 motion, and usually gape a Uttle at one extremity, — a 

 sure sign that their inhabitants live in the ground, and 

 do not roam from place to place. In the tribe oiMacro- 

 trachia, we thus have the pre-eminent type in the 

 Tellinidce, and the sub-typical in the MyadcE ; the 

 former being represented again by the Arcidce; and the 

 latter by the Aviculidce. We must, however, quit this 

 inviting train of generalisations, and proceed at once 

 to the only family of the Dithyra which stands in need 

 of a more detailed examination. 



(244.) The Unionidje, as before observed, are en- 

 tirely fluviatile, and are the only river shell-fish in 

 this tribe; except indeed the Etherirnr, by which they are 

 connected to the marine oysters. As they form that 

 particular family which unites the Atrachia to the 

 Macrotrachia, we naturally supposed, that when they 

 were better known, some trace of the siphons so cha- 

 racteristic of the latter tribe would be found in one of 

 the aberrant groups ; and in this expectation we have 

 not been disappointed. M. Deshayes, it appears, has 

 announced the fact, that the animal of Iridina pos- 

 sesses these tubes ; and it further appears that the major 

 part of the family, although really without these organs 

 in a permanent state, have yet the power of forming 

 two temporary tubes, with the posterior edge of the 

 mantle, which is fringed with cilia, serving to deter- 

 mine the direction of the water which flows into the 

 branchia.* Thus, whether we regard the shell or its 

 animal, we find that the natural station of the Unionida' 

 is determined by nature ; they are the last of the Atra- 

 chia as the ChamidcB are the first of the Macrotrachia. 



* Lea's Synopsis, p. 5. 



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