542 



Royal Society, 



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

 principles on which the subdivision of the group of Brachiopoda 

 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 the 

 family Terebratulida, 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 Terebratula and its allies, I beg to offer an account of my re- 

 sults to the Royal Society. 



The membrane which is commonly spoken of as ' the mantle/ and 

 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 

 the membrane lining the perforations in the shell (fig. lb); so that 

 their tubular caeca are, in fact, prolongations of the real external 

 surface of the mantle. The adhesion of the inner to the outer 



Fig. 1. 



Diagram of the intra-palleal sinus-system of Terebratula, with its caecal prolonga- 

 tions into the shell ; — A, B, section of valve ; a, inner layer of mantle, b, outer 

 layer in contact with the shell, and giving off caeca ; c, continuity of the two 

 at margin of valve. 



layer (which Professor Owen, not being aware of the existence of an 

 outer layer, interpreted as an adhesion of the mantle to the shell) 

 does not extend to the whole of the contiguous surfaces, but is 

 limited to certain bands or spots, — the two layers of membrane, in 

 the intervals between these, being separated by a set of irregular 

 spaces, freely communicating with one another, and with the cavities 



