the Skeletons of Mollusca, Crustacea and Echinodermata. 381 



nated. On examining a portion of it, however, which j\Ir. 

 Stutchbury submitted "to me, 1 was able most decidedly to as- 

 sert, that it possessed the intimate structure of the shell of the 

 Pectinidte, whdst the prismatic cellular structm-e and nacreous 

 lining of Avicula were altogether wanting. 



The prismatic cellular structure always occupies the exterior of 

 the shells in which it occm-s ; and their inner surface, as well as 

 the whole thickness of a large number of shells, is formed of 

 membranous shell-structm-e, by which I designate that structure 

 which seems to consist of alternatmg layers of carbonate of lime 

 and of the very delicate membrane already described. This mem- 

 brane is seldom or never /o/, however, but is folded into plications 

 like those of a frill, or corrugated into \ATinkles like those on 

 morocco leather, or arranged in other ways which cannot at pre- 

 sent be fully described, but which are characteristic of particular 

 famihes. These modes of arrangement determine the mode in 

 which the carbonate of lime is deposited, so that they become 

 perfectly evident when a thin section of a membranous shell is 

 examined with a microscope ; and they also determine the appear- 

 ance produced on fracture. Thus Mr. Gray has pointed out the 

 alternating directions of the rhomboidal crystals seen on frac- 

 tui-ing a porcellanous shell, such as Conus or Cijpi-cea ; and these 

 I believe to be entii*ely due to the alternating directions of the 

 membranous corrugations, which are shown by a section. In like 

 manner, the prismatic fibrous appearance, which is presented by 

 Mytilus, and by Septaj-ia {Teredo) gigantea, on fracture, is due to 

 the peculiarity in the form and dii'cction of the phcatious of the 

 membranous basis ; and the microscopic appearance of the latter 

 corresponds so precisely ^\ith that, which is to be seen in well- 

 preserved specimens of the fibro-calcareous sheath of the Belem- 

 nite, that I cannot but regard the structure of this interesting 

 fossil as completely analogous to that presented by the thick tube 

 of the recent Sept aria. 



There are two varieties of the membranous shell-structure, 

 which are produced by marked pecidiarities in the arrangement 

 of its organic base ; and which are so easily recognised by the mi- 

 croscope, as to become of great use in determining the characters 

 of fossil shells. The fii-st of these exists in nacre. That the na- 

 creous lustre is due to lines or striae, occm'ring ^^ithin certain di- 

 stances from each other, was long ago sho\\'n by Sir D. Brewster; 

 but my inquiries into the constitution of nacre have led me to a 

 \'iew of theii" cause somewhat different from his. Instead of re- 

 garding them as produced by the alternation of nmnerous layers 

 of membrane and calcareous matter, I have ascertained that they 

 are due to the plication or folding of a single layer, in such a 

 mode that the folds shall lie over one another in an imbricated 



