686 MOLLUSCA. 



layer of the shell is laminated, and is secreted by the whole outer 

 surface of the mantle. Hence, this stratum is produced in successive 

 layers throughout the entire life of the animal, each new layer ex- 

 tending a little beyond the one last formed ; and in this way are 

 produced the concentric " lines of growth," which are so character- 

 istic of the shells of the Bivalves. The calcareous laminae compos- 

 ing the inner layer of the shell are disposed approximately parallel 

 to the surface. In many cases (fig. 553, b) the laminae are very 

 delicate, and are more or less crumpled or undulated, so that when 

 the minute undulations of the successive layers of this stratum are 

 exposed on the surface, the rays of light are broken up in such a 

 way as to give rise to the iridescence and play of colours character- 

 istic of " mother-of-pearl " or " nacre." In other cases, however, 

 the laminae of this internal stratum are thicker, and are not minutely 

 undulated, and this layer then assumes a " porcellanous " character. 



As regards the chemical composition of the shell in the Bivalves, con- 

 siderable differences obtain as to the precise condition in which the 

 carbonate of lime presents itself. In some cases (as, for example, in 

 Ostrea), both the outer and inner layers of the shell are composed of 

 calcite, whereas in others both layers are formed of aragonite. In other 

 cases, again (as in Pinna), the outer prismatic layer is composed of cal- 

 cite, while the inner laminated layer is formed of aragonite. Owing to 

 the greater destructibility and instability of aragonite as compared with 

 calcite, these differences in composition lead to considerable differences 

 in the mode of preservation of different types of the Bivalves. Thus, 

 forms in which the shell is wholly composed of aragonite are often repre- 

 sented in the fossil state solely by moulds and casts ; while in those in 

 which the two layers of the shell differ in composition, the inner ara- 

 gonite-layer of the shell may have been wholly removed, while the outer 

 calcite-layer may be well preserved. 



Though the Bivalves agree with the Brachiopoda in possessing a 

 shell which is composed of two pieces or valves (small accessory 

 plates are present in Pholas, &c), there are, nevertheless, many 

 points in which the shell of a Lamellibranch is distinguished from 

 that of a Brachiopod, irrespective of the great difference in the 

 structure of the animal in each. The shell in the Brachiopoda, as 

 we have seen, is rarely or never quite equivalve, and always has its 

 two sides equally developed (equilateral) ; whilst the valves are 

 placed antero-posteriorly as regards the animal, one in front and 

 one behind, so that they are " dorsal " and " ventral." In the 

 Lamellibranchiata, on the other hand, the two valves are usually 

 of nearly equal size (equivalve), and are more developed on one 

 side than on the other (inequilateral) ; whilst their position as 

 regards the animal is always lateral, so that they are properly 

 termed "right" arid "left" valves, instead of "ventral" and 

 " dorsal." 



