206 Royal Society. 



brownish hue. Recollecting that Professor Owen, in his account of 

 dissections of some species of Terebratula and Orbicula (Transac- 

 tions of the Zoological Society, vol. i.), had spoken of an unusual 

 adhesion of the mantle to the shell in these Bivalves, it occurred to 

 me that this adhesion might be due to a continuity between the 

 mantle and these csecal tubuli ; and I carefully sought for evidence 

 of such a structure. In this, however, I was entirely unsuccess- 

 ful ; for the mantle, when stripped from the shell, presented no ap- 

 pearance whatever of having transmitted any such prolongations 

 into its substance ; on the contrary, it was evidently continued over 

 the mouths of the caeca with which it was in apposition ; and I fre- 

 quently found its external surface (^that in contact with the shell) 

 covered in patches with cells exactly resembling in size and aspect^ 

 those contained within the caeca. I was equally unsuccessful in th^ 

 attempt to trace any other connection between these caeca and thcP . 

 soft parts of the animal ; so that, although their importance in its 

 oeconomy scarcely admitted of doubt, the nature of their function re- 

 mained entirely unknown. The idea that they had any connection 

 with the formation of the shell itself, seemed to be completely nega- 

 tived by the fact, that in a large proportion of the group of Brachio- 

 PODA, no such perforations exist ; notwithstanding that their shells, 

 in every other feature of minute structure, are exactly accordant 

 with that of Terehratula. — The foregoing results were communicated , 

 to the British Scientific Association in 1847, and were embodied in^ 

 the Second Part of my " Report " published in its Transactions for? 

 that year. ., 



The physiological importance of the characters of ' perforation * 

 or * non-perforation ' has become continually more obvious, as thej 

 principles on which the subdivision of the group of Brachiopodsij^ 

 should be founded, have been gradually settled by those who have^ 

 concerned themselves with its systematic arrangement ; and in par-'? 

 ticular, the universal presence of the perforations in the shells of th^ 

 famWj TerebratulidcE, contrasted with their equally universal absence 

 in those of the family Rhynchonellida, unequivocally marked its rela-^ 

 tion to the general conformation of the animals of these subdivisions.*^ 



Having been requested by Mr. Davidson to undertake a more de-' 

 tailed investigation than I had yet made, into the minute structure., 

 of the shells of Brachiopoda, for the sake of throwing still further^ 

 light upon the classification of the group, I applied myself afresh? 

 to the solution of the problem, and believing that I have succeeded^ 

 in ascertaining the import of this curious feature in the organization* 

 of Terehratula and its allies, I beg to o^er an account of my re- ^ 

 suits to the Royal Society. ^J. 



The membrane which is commonly spoken of as ' the mantle/ an(J^ 

 which may be stripped from the shell by the use of sufficient force 

 to overcome its adhesions, must, I maintain, be considered as really 

 its inner layer only; for I find that an outer layer exists, so intimately ^ 

 incorporated with the shell as not to be separable from it without the 

 removal of its calcareous component by maceration in dilute acid. 

 When thus detached, this outer layer is found to be continuous with 



