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ACEPHALA PALLIOBRANCHIATA, 

 OE BRACHIOPODA. 



The researches of geologists, and the discoveries of 

 scientific travellers, jH-ove incontestably that in time and 

 space there are points or regions where peculiar groups 

 of organisms attain a maximum developement in variety 

 of character, or number of species, or multiplicity of in- 

 dividuals, constituting, as it were, a metropolis of the 

 family or genus. Before and after, in geological time, 

 and all around in geographical space, the number of mem- 

 bers of the generic type diminishes. The great section 

 of Mollusca, whose few living British representatives we 

 have now to describe, is a memorable example of this 

 phenomenon. The Brachiopods, though scantily distri- 

 buted through existing seas, abounded in those of the 

 long past, and rivalled the Lamellibranchiate bivalves in 

 numbers and variety, whilst the latter were poorly re- 

 presented by a very hw species, members of a very few 

 genera. Inferior in many features of their organization 

 to the LamelUhrancMata, in the main they must rank 

 as a great parallel group, equal in ordinal value, and 

 aberrant in some resj^ects from the Molluscan type. 



They are styled Palliobranchiata, because their respi- 

 ratory system, instead of being disposed in separate gills, 

 is combined with mantle, on which the vascular ramifi- 

 cations are distributed ; and Brachiopoda, because their 

 apparent organs of motion are two large, variously curved, 



