1[6 A POPULAR EXPOSITION OF THE 



mations. The known species obtained from the seas, lakes, and 

 rivers, of existing nature, somewhat exceed three thousand, whilst 

 nearly double that number of fossil species have been recognized. 

 These latter, however, belong it must be remembered, not to one 

 period, but to many successive epochs ; although on the other hand, 

 it is manifest that we see in them merely an incomplete record 

 of the lamellibranchiate fauna of the Past. 



In their classification, the lamellibranchiate mollusks fall into two 

 leading sections and four groups, as follows : 



(1) A-SIPHONIDA 



(1 a) Pleuroconcha. 

 (1 6) OrtJioconcha. 



(2) SiPHONIDA 



(2 a) Integro-PalUalia. 

 (2 h) Sinu'Pallialia. 



The animals of the first section are without the peculiar respira- 

 tory tubes possessed by the Siphonida. These latter, for example, 

 have a pair of short or long siphonal tubes, which assist in the pro- 

 cess of respiration, and which admit in the Sinu-pallialia of extension 

 beyond the shell. 



The Pleuroconcha, (group 1), of which the oyster may be taken as 

 a type, rest in their natural position with one valve below, and the 

 other above, and thus approximate to the Brachiopods. They have 

 in general but one large muscular impression 

 in the centre of the inside of each valve. This 

 forms a shallowpit, occupied by the muscle which 

 keeps the valves closed. The common fossil 

 known as Amhonychia radiata (fig. 110) may be 

 cited, though doubtfully, for its true afiinities 

 are still obscure, as an example of this division. 

 It is exceedingly abundant in the Hudson River 

 p. ^j. Group of the Lower Silurian series. 



The forms of the second group, or Orthocon- 

 cha, (as restricted above*,) are also without siphonal tubes, but their 



• The term Orthoconoha, it should be observed, is applied by some paleeontologists to our 

 groups, lb, 2 a, and 2 b, colleotively— the forms of the two first of these being united under 

 the subordinate group of Integro-PaUialia. 



