394 MOLLUSCOUS ANIMALS. 



the purpose. The remaining family of Terebratuliclce may be received 

 ■without question. It would be requisite before formally proposing such 

 a modification of the classification, to examine carefully all that is known 

 of the animals, or their shells, in a manner which my opportunities here 

 do not admit. I can therefore, at present, only express my expectation 

 that the five tendencies, usually manifested under each class, or sub- 

 class, will be observed here, as elsewhere, and that corresponding truly 

 natural divisions will thus be formed. The Palliobranchiata are most 

 of them known to us in a fossil state. The few recent species assist us 

 in rightly interpreting the appearances they present. 



We advance to the consideration of the Lamellibranchiata, the 

 higher sub-classes of Conchifera, and one of the most extensive 

 and important divisions of the molluscous sub-kingdom. It is a 

 division in which the families have been worked out with consi- 

 derable success, though the distinction between the greater families 

 and sub-families has not always been duly attended to, but the com- 

 bination of the families into orders has been either neglected, or per- 

 formed in a manner that is far from satisfactory. La Marck proposes 

 two orders : Monomyaria and Dimyaria, founded on the number of muscu- 

 lar impressions on the shell. It is not to be doubted that the single muscle 

 marks inferior development, and is a useful character in determining 

 lower groupes ; but it may be greatly doubted whether it is an absolute 

 character, sufficient to limit an order, and marking in itself a grand di- 

 vision among Lamellibranchiate Mollusks. It would not be allowable to 

 place Tridacna in a different order from Ghana, on account of its blen- 

 ded sub-central muscular impressions, nor to remove My til idee from th« 

 lowest order, and from the near neighbourhood of Avicididue, on account 

 of the presence of two almost combined muscular impressicis, nor is it 

 by any means proved that Arcadce, which in several particulars seem to 

 belong to the lowest division, must be separated from it on account of 

 their two muscular impressions : much less can it be maintained that 

 the Dimyaria have not divisions among themseles, on other grounds quite 

 as important as their distinction from Monomyaria, and therefore increas- 

 ing the number of orders. Another proposed leading division of La- 

 mellihranchata is into Asiphonida and Siphonida. This depends on the 

 entire separation, or union in some part of their margins, of the mantle 

 lobes, leaving distinct openings for the incurrent and excurrent water 

 >?hich is drawn in by the action of cilia, and after providing both for 

 nutrition and aeration is expelled through the other orifice. It is to be 



