CHAP. X. MACROTRACHIA AND ATRACHIA. 257 



by means of Lima and Pecten. Finally, in order to 

 close this circle, we have the Etherid^, which, with 

 Mulleria, " is remarkable as a group possessing in 

 itself both the character of being intermediate between 

 Etheria and Ostrea, and as apparently connecting the 

 regular freshwater bivalves with the regular marine 

 bivalves, as Ostrea.* The sinus at the posterior ex- 

 tremity of the hgament of 3fuUeria resembles the 

 Unionidce and the Etheria; while in its single mus- 

 cular impression, as well as its general form, it ap- 

 proaches to Ostrea." This obvious affinity at once 

 closes the circle of the Atrachia, by the series re- 

 turning to the Unionidcp. The following table will 

 consequently exhibit the mutual relations of both these 

 tribes: — 



Analogies of the Macrotrachia and the Atrachia. 



Families of the A„r,JnaiPi Families of the 



Macrotrachice. Analogies. Atrachia. 



rrr^.^.^r.r.^ fShells veiv compact, the valves 7 . 



Tellimd/e. ^ ^1^3^^ 0- F > j Arcaid^e. 



Mtad^. Valves gaping at one extremity. Aviculid^. 



TJ„^r.T>™, C Animals affixed to, or living with- > ^ 



Pholid^. ] in, other substances. j Ostreid^. 



SAXiCAViDiE. Shells irregular, without teeth. Etherid^. 



^ r Lateral teeth long, linear, only on 7 r^ 



Chamid^. J one side of the bosses. j I^nionid^. 



(242.) It will be seen, by the subsequent arrangement 

 of the genera, that they follow each other precisely in the 

 same way, so as to represent the families. To follow 

 out this exposition, however, in the present tribe, as 

 we have already done in that of the Gasteropoda, is 

 manifestly impossible ; but, as we state this fact as the 

 result of a careful analysis of the whole, the reader, 

 knowing the general principle, may apply it to any 

 particular group, whose analogies he wishes to trace. 

 As we shall enter into a full investigation of the Unio- 

 nidce, or river pearl-shells, our remaining space will 



* Sowerby's Genera of Shells, art. Mulleria. 



