2 REPORT — 1844. 



quiries into the structure of the sliells of Mollusca, Crustacea, and Echinoder- 

 mata ; -which I have since been prosecuting as time and opportunity have been 

 afforded me. About the same period, Mr. Bowerbank connnenced an inde- 

 pendent series of observations; which have had reference, however, I'ather to 

 the formation of shell, than to its microscopic characters when complete ; and 

 which have been limited to a comparatively small number of species, whilst 

 my own have included a very extensive range. Finding that our paths of in- 

 quiry were so distinct, Mr. Bowerbank and I agreed to pursue them inde- 

 pendently of each other ; and the results of our researches were simul- 

 taneously communicated, — on his part to the Microscopical Society, — and on 

 mine to the Royal Society, — in January 1843. A brief sketch of my own 

 inquiries was laid before the British Association at its Cork meeting ; and, 

 with the aid of the grant which was then made to me from the funds of the 

 Association, together with the assistance I have received from various quar- 

 ters, in regard to the collection of subjects for examination, — especially from 

 the Geological Society, the Council of which has liberally permitted me to 

 examine duplicate specimens from its valuable museum, and from Messrs. H. 

 Cuming, S. Worsley, S. P. Pratt and J. Morris, — I have made during the past 

 year little short of a thousand preparations of shell-structure. A considerable 

 part of my labour has been directed to the determination of the questions, — 

 whether an uniform structure prevails through every part of the same shell, 

 so that the structure of the whole shell may be predicated from that of a 

 small portion of it, — and whether the same structure is found in different in- 

 dividuals of the same species, and among different species of the same genus. 

 It is obvious that a settlement of these questions must be of great importance 

 in the application of the Microscope to the determination of fossil shells ; and 

 I think that I am now entitled to answer them with some degree of confi- 

 dence. 1 have, in a considerable number of instances, submitted every por- 

 tion of a shell to microscopic investigation, selecting such specimens as, from 

 the peculiar characters of their structure, would serve as types to which 

 to refer others ; and I have invariably found that an uniform structure 

 pervades the whole of each ; so that the examination of but a very small 

 fragment is sufficient to determine the structure of the entire shell. I feel 

 equally certain with respect to the correspondence between the structure of 

 different individuals of the same species ; as I have never found any decided 

 variation, although I have in some instances examined several specimens of 

 one kind. With respect to the degree of difference which may exist among 

 the several species of the same genus, I am not yet prepared to speak with 

 certainty. In general I have found the correspondence such, that the size of 

 the elementary parts is the chief point of difference ; but occasionally I have 

 found particular forms of structure present in one species and absent in 

 another. It will hereafter appear, however, that this difference corresponds 

 with other variations, which are probably to be considered as establishing 

 generic distinctions in the cases in question. 



In the following Report, it is my intention to give a general account of the 

 chief forms of elementary structure, which I have met with in Shell ; and to 

 enter into systematic details in regard to the group of Brachiojjoda, and the 

 families of Placunidce, Ostracea;, Pectinida, Margaritacece, and Unionida, 

 among the Lamellibranchiate Bivalves. The remaining families of Bivalves, 

 and the whole group of Univalves, must be reserved for a future report. 



I am desirous that it should be understood that, where I do not express 

 myself to the contrary, my statements are the result of my own researches ; 

 and that I am ready to substantiate them by reference to the preparations on 

 which they are grounded, all of which are in my possession. 



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