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Ch IX.] AT SUCCESSIVE GEOLOGICAL PERIODS. 



animals and plants inhabiting the land, the lake 



151 



and tlie 



marine mollusc 



articulata, and radiata. Our knowledge, therefore, of the 

 living creation of any given period of the past may be said 



common 



some 



may modify 



all our previous generalisations. 



Mollusca. 



mollusca 



the most important in geology, as, owing to the durable 



mor 



m 



creatures. They are also peculiarly well-fitted to throw light 

 on the controverted question whether there has or has not 



been a gr 



sim 



mor 



from 



structure. 



rades of 

 ls meant 



one in which there are a greater number of organs specially 

 devoted to particular functions. Thus in the lowest divisions, 

 such as the Bryozoa and Brachiopoda, we find no separate 



locomotion 



lamellibranchiate bivalves, although they are without heads, 

 we find a heart, gills, a foot, and several other organs, want- 

 ing in the inferior orders before alluded to. The gasteropoda, 

 acmin, have a head, mouth, teeth, a special breathing appa- 



them 



m 



struments appointed to perform distinct functions, such a 

 concentration of the nervous system in what may be regarded 

 as a brain, such acuteness of the senses, especially that 



locomotion, that we cannot 



of sight, with such powers ._ _. 



refuse to assign to them a place superior to that of all the 



mollusca. and even to some 



although these last belong to a type which as a whole ranks 



****** * * M -* 



mUCh *** & ** V * XXX ,XXV .V.XV. 



have now, therefore, to enquire whether the fossil 



We 



. . _ — . — . y 7 __ — ± 



representatives of the different divisions of the mollusca above 

 enumerated, the Bryozoa, Brachiopoda, Lamellibranchiata, 

 Gasteropoda, and Cephalopoda, made their appearance in 



