CLASSES OF THE MOLLUSCA. 



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 pui-poses 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 an'angement" is a perpetual bar to advancement, " con- 

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



i 



Fig. 7. A Bivalve* 



Fig. 8. A Tunicary .J 



Mya tnmcata, L. \. From Forbes and Ilanloy. 

 t Ascidia mentuJa, Miill. Ideal reprcsentatiou ; from a specimen di-edaed 

 by Mr. Bowerbaiik, oft' Tenby. 



B Z 



