ON THE MICROSCOPIC STRUCTURE OP SHELLS. 3 



I shall commence with a brief outline of the researches and conclusions of 

 Mr. Hatchett (Phil. Trans, 1799), and of Mr. Gray (Phil. Trans. 1833) ; the 

 only two original inquirers on this subject, so far as I am aware, since the 

 time of Reaumur. 



The experiments of Mr. Hatchett led him to divide Shells into two classes, 

 the porcellanous and the nacreous. He stated that those belonging to 

 the former group are composed of carbonate of lime, held together by so 

 small a proportion of animal matter, that, although its presence may be recog- 

 nized by the eifects of heat upon the shell, no membranous film is left after 

 the action of dilute acid upon it. Under the nacreous group he placed those 

 shells which, though they do not all exhibit the nacreous lustre, possess an 

 amount of animal membrane sufficiently great for the form of the shell to be 

 more or less perfectly preserved, after the calcareous matter has been com- 

 pletely dissolved away by dilute acid. To such shells the term memhranous 

 has been subsequently applied with much greater propriety ; and of the class 

 of membranous shells, the true nacreous form a subordinate division. This 

 distinction, however, cannot now hold good ; since all shells, without excep- 

 tion, have a distinct animal basis, as will be shown hereafter. 



According to Mr. Gray, another classification of Shells may be founded 

 upon the manner in which the carbonate of lime is deposited in their sub- 

 stance; some shells exhibiting a distinctly crysto^/«we fracture, whilst others are 

 granular or concretionary, Mr. Gray states that, among the crystalline shells, 

 some may be found, in which the carbonate of lime exhibits a rhomboidal 

 crystallization, whilst in others it is prismatic. I think it will appear from my 

 inquiries, that the calcareous matter in all shells is nearly equally crystalline 

 in its aggregation ; and that the parti cularybrTWS which their fracture presents 

 are determined, chiefly if not entirely, by the arrangement of the animal basis 

 of the shell, which possesses a more or less highly organized structure. 



I shall now proceed to describe the principal varieties of structure which 

 I have met with in the examination of upwards of 400 species of Shells, recent 

 and fossil, selected from all the principal families of Mollusca. When exami- 

 ning recent shells, I have, in nearly every instance, submitted them to micro- 

 scopic investigation in at least two ways ; first, by making thin sections of 

 them, so that their structure might be examined by transmitted light ; and 

 second, by examining the animal membrane left after the removal of the cal- 

 careous matter by dilute muriatic acid, which I shall name for convenience 

 the decalcifying process. In many instances also, I have found the examina- 

 tion of the natural or fractured surfaces of the shell by reflected light, or of 

 the thin lamins into which many shells will readily split, to afford valuable 

 information. These methods of investigation mutually aid and correct each 

 other ; and neither can be prosecuted alone, without much liability to error. 



n. On the Condition of the Calcareous Matter in Shell, 



1. All thin sections of recent Shell are translucent, except those which con- 

 tain a large amount of opake colouring matter, or which (as sometimes hap- 

 pens) have a layer of calcareous particles deposited in a chalky or concre- 

 tionary state between the proper laminae of shell-structure. This is the case 

 in the common Oyster, as pointed out by Mr. Gray ; and in many other shells 

 which possess an opake white aspect, such as Fusus despectus. But I can- 

 not regard such layers as forming part of the proper structure of the shell ; 

 since the particles of carbonate of lime, of which they consist, are not con- 

 nected by any organic basis. 



2. Again, all thin sections of shell possess the power of depolarizing light, 

 so that the portion of shell appears bright upon a dark ground, when the 



