SHELLS OF BRACHIOPODS AND GASTEROPODS. 



603 



Sections of the shell. — In the Families Spiriferidre and Stropho- 

 menidce, on the other hand, some Species possess the perforations, 

 whilst others are destitute of them ; so that their presence or 



Fig. 314. 



Fig. 315. 



Fig. 316. 



Fig. 314. Horizontal section of Shell of Terebratula bull at a (fossil, Oolite). 

 Fig. 315. Ditto .... of Megerlia lima (fossil, Chalk). 

 Fig. 316. Ditto .... of Spiriferina rostrata (Triassic). 



absence there serves only to mark-out subordinate groups. This, 

 however, is what holds-good in regard to characters of almost 

 every description, in other departments of Natural History ; 

 a character which is of fundamental importance from its 

 close relation to the general plan of organization in one group, 

 being, from its want of constancy, of far less account in 

 another.* 



467. There is not by any means the same amount of diversity 

 in the structure of the Shell in the class of Gasteropoda ; a certain 

 typical plan of construction being common to by far the greater 

 number of them. The small proportion of Animal matter con- 

 tained in most of these Shells, is a very marked feature in their 

 character ; and it serves to render other features indistinct, since 

 the residuum left after the removal of the Calcareous matter is 

 usually so imperfect, as to give no clue whatever to the explanation 

 of the appearances shown by sections. Nevertheless, the structure 

 of these shells is by no means homogeneous, but always exhibits 

 indications, more or less clear, of a definite arrangement. The 



* For a particular account of the Author's researches on this group, 

 see his Memoir on the subject, forming part of the introduction of Mr. 

 Davidson's "Monograph of the British Fossil Brachiopoda," published by 

 the Palaeontographical Society. — A very remarkable example of the im- 

 portance of the presence or absence of the perforations, in distinguishing 

 Shells whose internal structure shows them to be Generically different, 

 whilst from their external conformation they would be supposed to be 

 not only generically but specifically identical, will be found in the "Annals 

 of Natural History," Ser. 3, Vol. xx. (1867), p. 68. 



