4 REPORT — 1844. 



polarizing and analysing plates or prisms are so arranged as to prevent the 

 transmission of ordinary light. 



3. From these facts I think we are entitled to conclude, that the calcareous 

 matter of shell is in a state of crystalline aggregation, even when no crystal- 

 Mne forms are presented by it. The absence of the latter is probably due to 

 the mode in which the calcareous matter is set free from the whole surface at 

 once ; so that there is not room (so to speak) for these forms to be generated. 

 This conclusion is strengthened by the remarkable fact, that crystalline forms 

 do present themselves under peculiar circumstances. Thus I have met, in the 

 Oyster, with layers incompletely calcified ; so that, instead of being covered 

 by a continuous and uniform deposit of carbonate of lime, the membrane was 

 studded with a multitude of minute rhomboidal bodies, varying in size from 

 about the l-6000th to the l-2000th of an inch across (fig. 16) ; the effect of 

 polarized light and of chemical reagents upon which, left no doubt that they 

 are crystals of carbonate of lime*. In very thin sections of parts of Cypraea 

 and other porcellanous shells, in which the quantity of animal matter is ex- 

 tremely small, I have frequently seen the apparently-homogeneous calcareous 

 deposit crossed by lines, inclined to each other in such a manner, as to indicate 

 a rhomboidal crystallization in its substance. And in the tooth of Mya 

 aretiaria, I have seen an appearance which seems to me (from a comparison 

 of it with numerous allied forms of structure) to indicate the crystallization 

 of the carbonate of lime in a radiating manner, (the centres being the nuclei 

 of the cells, within which each group of crystals was originally inclosed,) 

 somewhat after the manner of i-adiating Arragonite or Wavellite (fig. 14). 



III. Of the Animal Basis of Shell. 



4<. When a portion of any recent Shell is submitted to the decalcifying 

 process, a perfectly definite animal basis remains. Tiiis basis may be nothing 

 more than a film of memhrane, so delicate as almost to elude detection f, but 

 evidently not an amorphous residuum ; or it may be a membrane of firmer 

 consistence, presenting regular plications or corrugations ; or it may consist 

 of an aggregation of cells, having very definite membranous walls, and a 

 more or less regular form. My first division of shell-structures, therefore, is, 

 according to the character of the animal basis, into the cellular and the mem- 

 branous ; these I shall now proceed to describe in detail. 



IV. Prismatic Cellidar Structure. 

 5. If a small portion be broken away from the thin margin of the shell of 

 any species of Pinna, and it be placed without any preparation under a low 

 magnifying power, it presents on each of its surfaces, when viewed by trans- 

 mitted light, very much the aspect of a honeycomb ; whilst at the broken 

 edge it exhibits an appearance M'hich is evidently fibrosis to the eye, but 

 which, when examined under the microscope with reflected light, resembles 

 that of an assemblage of basaltic columns. The shell is thus seen to be com- 

 posed of a vast multitude of prisms, having for the most part a tolerably 

 regular hexagonal shape and nearly uniform size. These are arranged per- 

 pendicularly (or nearly so) to the surface of each lamina, so that its thick- 

 ness is formed by their length, and its two surfaces by their extremities. A 

 more satisfactory view of these prisms is obtained by grinding down a lamina, 



* It is stated by Wagner, that minute crystals of calcareous matter are to be found in the 

 cartilaginous envelope of Ascidia mammiUata. — (Lelirbuch der vergleicbenden Aiiatomie, 

 p. 60.) 



t When such films have not been visible in the menstruum, I have found them involved in 

 the bubbles that lay on the smface after the effervescence was over. 



