CLASSES OF THE MOLLUSCA. 



9 



to see ; and here, as in his botany, employed an artificial, in pre- 

 ference to a natural method. 



The systematic arrangement of natural objects ought not, 

 however, to be guided by convenience, nor '' framed merely for 

 the purposes of easy remembrance and communication." The 

 true method must be suggested by the objects themselves, by 

 their qualities and relations ; — it may not be easy to learn, — it 

 may require perpetual modification and adjustment, — but inas- 

 much as it represents the existing state of knowledge it will aid 

 in the understanding of the subject, whereas a '' dead and 

 arbitrary arrangement" is a perpetual bar to advancement, " con- 

 taining in itself no principle of progression." {Coleridge.) 



I 



Fig. 7. A Bivalve.* 



Fig. 8. A Tunicary.^ 



Mya truncata, L. \. From Forbes and Hanlej. 

 t Ascidia mentula, Miill. Ideal representation; from a specimen dred^^-ed 

 by Mr. Bowerbank, off Tenby. 



B 3 



