OF CONCHOLOGY. 41 



of all their structural characters ; few would assert that such 

 study would reveal sharp lines of demarkation between verte- 

 brates and invertebrates, mollusks and annulosa, radiates and 

 protozoans. On the contrary, we know that with the very lim- 

 ited knowledge which we possess of the forms inhabiting the 

 earth at the present period, there are such close approximations 

 between members of the different great groups as to render it a 

 matter of great difficulty to refer them to their proper places in 

 the natural system. This difficulty has seemed so insuperable to 

 some of the most eminent naturalists of the present day, that 

 they have entirely rejected the great groups (representing differ- 

 ent fundamental types of structure to Cuvier) and have substi- 

 tuted a larger number of smaller ones ; and even of some of these 

 small groups the boundaries are a matter of discussion. 



Under these circumstances, we should naturally expect to find 

 among the older types of animal life, a more generalized struc- 

 ture than in those of later development, and that the living rep- 

 resentatives of these old types should be among those which offer 

 the greatest difficulties in referring them to a definite place in 

 our systems. 



For my own part I confess I incline to the belief that, after 

 eliminating the Protozoa, the revised sub-kingdoms or embranch- 

 ments of Cuvier have still a meaning and a value, though not 

 such as he ascribed to them. Among the lower invertebrates, 

 however, there are many forms which, from their simplicity, offer 

 few characters ; and in such cases we can only strike the bal- 

 ance and refer them to places indicated by the majority of their 

 characteristic features. This process must always depend, to some 

 extent, upon the individual opinion of the particular student, and 

 while the majority of naturalists may be united upon a given 

 question, there may be others who view the subject differently 

 upon the same evidence. It does not appear to me that the 

 brachiopods can be included in this category of doubtful forms. 

 Indeed, their class and ordinal characters seem to indicate their 

 position with sufficient clearness to admit of no doubt upon the 

 subject. 



Before proceeding to discuss this point, however, I shall define, 

 to the best of my ability, the groups under consideration, and 

 then show, as far as may be, the grounds upon which I consider 

 the brachiopods to belong to one, rather than to another group ; 

 after which I propose to take up separately each of Prof. Morse's 

 supposed homologies, and sift it as thoroughly and impartially as 

 possible. In the first place, the Mollusca, including both the 

 typical forms and the Molluscoidea, appear to present the follow- 

 ing characters as typical features of structure : 



