40 INTRODUCTION. 



tubular structure which is characteristic of the group, yet its absence is fully accounted 

 for by the metamorphic condition of the shell, a like obliteration having occurred in fossil 

 Discincs and Lingulce. 



In bringing to a conclusion the present account of the structure of the shell in this 

 most interesting group of Mollusca, I cannot but express my regret that it has not lain in 

 my power to render it more complete. Those who seek for assistance in classification 

 from the characters which I have furnished, will, I fear, complain with much reason of the 

 small proportion of species which I have submitted to examination, especially in some of 

 those generic types (as Spirifer, Athyrls, and Orthis) whose variability in this respect 

 makes such an examination especially important. I may be permitted to remark, however, 

 that the labour of preparing microscopic sections (the only mode, as I have shown, which 

 can give satisfactory results in regard to fossil Brachiopods) is by no means trifling, and 

 that the results here given are based on the examination of no fewer than three hundred 

 such preparations. No doubt, I venture to think, can now remain as to the importance, 

 as a natural-history character, of the perforation or non-perforation of the shell, for the 

 reception of the vascular processes of the animal, — a feature in its structure which I may 

 claim to have been the first to demonstrate ; and I must leave the more extensive 

 determination of it, as regards individual species, to be carried out by those who possess 

 more time and opportunity for such a work than I can command. 



