922 MOLLUSC A AND BRACHIOPODA 



their shape to the deposit of carbonate of lime formed within them. 

 The progress of inquiry, however, has led to an important modifica- 

 tion of this interpretation, the Author being now disposed to agree 

 with Huxley l in the belief that the entire thickness of the shell 

 is formed as an excretion from the surface of the epidermis, and 

 that the horny layer which in ordinary shells forms their external 

 envelope or ' periostracum,' 2 being here thrown out at the same time 

 with the calcifying material, is converted into the likeness of a 

 cellular membrane by the pressure of the prisms that are formed by 

 crystallisation at regular distances in the midst of it. The pecu- 

 liar conditions under which calcareous concretions form themselves 

 in an organic matrix have been carefully studied by Mr. Rainey 

 and Dr. W. M. Ord, of whose researches some account will be given 

 hereafter. 



The internal layer of the shells of the Margaritacece and some 

 other families has a ' nacreous ' or iridescent lustre, which depends 

 (as Sir D. Brewster has shown 3 ) upon the striation of its surface 

 with a series of grooved lines, which usually run nearly parallel to 

 each other (fig. 697). As these lines are not obliterated by any 

 amount of polishing, it is obvious that their presence depends upon 

 something peculiar in the texture of this substance, and not upon 

 any mere superficial arrangement. When a piece of the nacre (com- 

 monly known as ' mother of- pearl ') of the Meleagrina or ' pearl-oyster ' 

 is carefully examined, it becomes evident that the lines are produced 

 by the cropping out of laminae of shell situated more or less obliquely 

 to the plane of the surface. The greater the dip of these laminae, the 

 closer will their edges be ; whilst the less the angle which they make 

 with the surface, the wider will be the interval between the lines. 

 When the section passes for any distance in the plane of a lamina, no 

 lines will present themselves on that space. And thus the appearance 

 of a section of nacre is such as to have been aptly compared by Sir J. 

 Herschel to the surface of a smoothed deal board, in which the woody 

 layers are cut perpendicularly to their surface in one part, and nearly 

 in their plane in another. Sir D. Brewster (loc. cit.) appears to have 

 supposed that nacre consists of a multitude of layers of carbonate of 

 lime alternating with animal membrane, and that the presence of 

 the grooved lines on the most highly polished surface is due to the 

 wearing away of the edges of the animal laminae, whilst those of the 

 hard calcareous laminae stand out. If each line upon the nacreous 

 surface, however, indicates a distinct layer of shell-substance, a very 

 thin section of ' mother-of-pearl ' ought to contain many hundred 

 laminae, in accordance with the number of lines upon its surface, 

 these being frequently no more than ^^th of an inch apart. But 

 when the nacre is treated with dilute acid, so as to dissolve its cal- 



1 See his article, ' Tegumentary Organs,' in Cyclopedia of Anatomy and 

 , supplementary volume, pp. 489-492. 



The periostracum is the yellowish-brown membrane covering the surface of 

 many shells, which is often (but erroneously) termed their epidermis. 



5 Phil. Trans. 1814, p. 397. The late Mr. Barton (of the Mint) succeeded in 

 producing an artificial iridescence on metallic buttons by drawing closely approxi- 

 mating lines with a diamond point upon the surface of the steel die by which they 

 were struck. 



