CHAPTER II. 

 ON THE INTIMATE STRUCTURE OF THE SHELLS OF BRACHIOPODA. 



BY PROFESSOR CARPENTER, M.D., F.R.S., &c. &c. 



The shells of the Brachiopoda, whether recent or fossil, present most interesting 

 subjects for microscopical investigation ; their structure being, in almost every instance, 

 quite distinct from that of the shells of the Lamellibranchiata, or Gasteropoda, and being 

 so characteristic, that even minute fragments may be referred with certainty to this group, 

 unless their texture has been altered by metamorphic action. Such action appears to have 

 so constantly taken place in the shells preserved in the Palaeozoic rocks, that, with regard 

 to some of those genera which are limited to that series, such as Pentamerus, Calceola, or 

 Producfus, it is impossible to say with positive certainty how far their structure is 

 conformable to the general type of the group, since this is but very obscurely shadowed 

 forth in them. But, since we find Terebratula, Pliynconella, and Spirifers of those 

 formations presenting the very same deficiency in characteristic structure with that which 

 is encountered among the genera previously named, we seem fully entitled to affirm, that 

 the absence of such structure in the fossils of the strictly-Palaeozoic genera is not to be 

 taken as any indication that the living shells departed from the general type ; whilst, on 

 the other hand, the perfect conformity to that type, of such structure as can be dis- 

 tinguished in them, ih a positive indication that the shells of these genera were constructed 

 upon the same plan with those of the existing Brachiopoda, as their other affinities would 

 render probable. The following statements, then, which are chiefly based on the 

 examination of recent Brachiopoda, may be considered to apply equally to the extinct, 

 until any proof shall have been given to the contrary. 



There is not, in the shells of Brachiopoda, that distinction between the outer and inner 

 layers, either in structure or mode of growth, which prevails among the ordinary 

 bivalves ; and it seems obvious, both from the nature of the shell substance, and from the 

 mode in which it is extended, that the whole thickness of the Brachiopod shell corresponds 

 with the outer layer only in the Lamellibranchiata, (PL IV, fig. 1.) For, as will be 

 presently shown, it must be regarded in the light of a calcified epidermis, like the 

 prismatic external layer of Pinna or Avicula -^ and it is augmented by an addition to its 



1 See the author's Memoir on the Microscopic Structure of Shells, in the Report of the British 

 Association for 1844, p. 6. 



