124 



to the edge of the old lip of the shell, and becoming indurated by 

 the action of the air, formed a cartilaginous mould and basis upon 

 which the calcareous matter was to be ultimately deposited. The 

 carbonate of lime thus destined to afford strength and substance to 

 the new shell, it was believed, was secreted in the foot or mantle of 

 the animal, whence it was poured forth, and applied mechanically to 

 the surface of the membrane, layer after layer being thus deposited 

 from within, until the shell became of the required degree of thick- 

 ness : thus supposing a perfectly mechanical or crystallized arrange- 

 ment of the earthy matter of the shell. And in the latter of these 

 errors naturalists were confirmed by the Compte de Bournon's inves- 

 tigations of shelly structures, described in the last four chapters of 

 his ' Traite de Mineralogie,' and figured in his plate 1.* 



I have not had an opportunity of repeating with sufficient care the 

 observations made by Reaumur upon the animal during the progress 

 of the development of the new lip of the shell, but I believe him to 

 be correct in his description of the first stage of the process, which is 

 quite in accordance with the general views entertained by modern 

 physiologists of the origin and mode of repair of primary membranous 

 tissues. My own observations on the formation of the new lip have 

 been confined to a careful examination of the newly-formed mem- 

 branes, and the results are as follow. 



If the newly-formed lip be carefully removed, so as to include a 

 portion of the shelly matter as well as the membrane, and be put into 



* By using an inadequate amount of microscopical power, this learned mineralo- 

 gist has evidently been induced to believe that the beautiful plates composed of 

 single series of elongated cells, crossing each other at right angles to their respective 

 axes, and which I shall hereafter describe in treating of the structure of the univalves, 

 were the result of crystallization ; and under this impression he says, page 310, " Mon 

 but est de faire voir ici, que ce tet appartient exclusivement a la cristallisation de la 

 chaux carbonatee. Que l'animal, par les contours de son corps, et les diverses exten- 

 sions et courbures de ses membranes, a tres probablement determine la forme que ce 

 tet presente ; mais que, quant a sa nature, c'est un chaux carbonatee simple, cristal- 

 lisee a l'etat lamelleux et par couches, ainsi que celle qui donne naissance aux depots 

 cristallins et autres varietes analogues de cette substance. Si on en excepte, en erTet, 

 les parties dans lequelles une tres-legere dose de gelatine a ete placee par l'animal, 

 pour y servir de matiere colorante, et y orner, comme telle, la surface d'un tres-grand 

 nombre d'entre elles, le reste renferme une dose si foible de cette meme gelatine, qu'il 

 doit etre regarde comme appartenant a une des varietes les plus pures de la chaux 

 carbonatee : il en existe en realite bien peu, meme parmi les varietes en cristaux 

 transparens et determines, qui ne soient melangees d'une dose plus considerable de 

 substances etrangeres.'' 



