40 SHELLS AND SHELL-FISH. PART I. 



distinctions which nature has thus stamped upon the 

 primary groups,, infinitely diversified as they are in all 

 other respects. 



(35.) Our next object is, to show upon what grounds 

 our conviction rests, that these orders form a circular 

 group,, more or less perfect. It is hardly necessary to 

 apprise the zoologist, that in this class of animals, above 

 all others,, the effects of those revolutions in our globe, 

 which have nearly exterminated so many races of animals, 

 are most apparent. It appears, at first, difficult to say 

 whether this destruction has been most extensive 

 among those whose hard vestiges yet remain in a fossil 

 state, or among such as, being entirely soft, have alto- 

 gether disappeared, and "left not a wreck behind." 

 Our own opinion, however, is decidedly in favour of 

 the latter supposition. Four fifths of the testaceous 

 Cephalopoda have certainly disappeared ; yet their shelly 

 portions, abounding in incalculable quantities in all the 

 older formations, are yet sufficient, with the aid of the 

 few species now existing, to give us some general idea 

 of the chain of continuity : but, among the Nudibran- 

 chia and the Branchiopoda, whole families seem to have 

 disappeared : such of the latter, indeed, as possessed a 

 hard covering may be judged of by their shells; but 

 in what way their union was effected with the existing 

 races of Dithyra it is altogether impossible to judge. 

 The Nudibranchia, again, are allowed by all naturalists 

 to constitute a distinct tribe ; and yet it does not con- 

 tain more individuals, or a greater diversity of structure, 

 than may be met with in any one of the numerous 

 families of the gastropod Testacea. Now, it must be 

 remembered that these are all soft, and often sub- 

 gelatinous, animals, without any hard or durable parts 

 whatsoever ; so that all that have only existed in former 

 periods of our globe, have been exterminated both in 

 life and substance, no indication whatever remaining 

 that they ever had existence. Upon what ground, 

 then, it may be asked, do we take for granted that they 

 have ever been among the things that were ? and what 



