12 



POPULAR CONCHOLOGY. 



by a succession of thin strata of liquid shelly matter, which 

 exude from that portion of the mantle which has been laid 

 bare. 



Dr. Carpenter has given a most interesting account of 

 the structure of shell as shown by the microscope. In 

 some classes there is a prismatic formation of membrane, 

 like honeycomb, the interstices filled with carbonate of 

 lime ; in others there is a membranous structure, con- 

 sisting of alternate layers of membrane and carbonate of 

 lime ; these two modes of construction exist also in the 

 same shell, the former being the external portion, and 

 the other the internal; Dr. Carpenter describes other 

 formations, all of which are highly curious. 



The regularity in the growth of many shells appears to 

 be interrupted at particular times by changes which arise 

 in the shape of the mantle ; this is observable in several 

 species, which exhibit knobs or spines projecting from their 

 surface, and which have been produced by the mantle alter- 

 ing its form, and dividing into thick or slender branches. 

 On these processes are formed hollow spines, which are 

 gradually filled up by successive depositions of shelly 

 matter, and eventually become solid ; the mantle, being 

 then withdrawn from them, proceeds in the formation of 

 new shell, branching out at regular intervals into another 

 row of spines, knobs, or foliations ; and in this manner are 

 produced the prickles, ridges, and other excrescences 

 visible in many genera, the Murex, for example. Analo- 

 gous to these variations in the deposit of shell by the 

 mantle, but differing, inasmuch as they only occur at one 

 period, are the developements which take place when the 

 mollusc has arrived at its full growth. In the adjoining- 

 cut are represented the immature and mature states of 

 the same shell (Ptcroccras la?7ibis). It will be seen that 

 they differ widely; and the variation has been caused by 



