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ACEPHALA PALLIOBRANCHIATA, 

 OR BRACIIIOPODA. 



The researches of geologists, and the discoveries of 

 scientific travellers, prove incontestably that in time and 

 space there are points or regions where peculiar groups 

 of organisms attain a maximum devclopement 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 few species, members of a very few 

 genera. Inferior in many features of their organization 

 to the Lamellibranchiata, in the main they must rank 

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

 aberrant in some respects 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, 



