Conchology. 171 



is much thicker than when the growth was gradual and impercepti- 

 ble, and consequently the place at which the growth commences, 

 after a long interruption, is distinguished by a more elevated ridge, 

 than in the continual progressive additions which it receives. 

 The numerous instances of this interruption in the growth of 

 shells, will occur to the attentive conchologist in the progress of 

 his researches. We know of a cabinet containing a fine illustra- 

 tion of the same thing, in a specimen of JVIurex ramosus. The 

 animal, it would appear from the original part of the shell, had 

 been for some time in a sickly or unhealthy state; for it has un- 

 dergone many of the changes to which dead shells are subject. It 

 has lost its enamel; it seems to have undergone some degree of 

 decomposition, and some species of serpula and other parsaitical 

 animals had made it their abode; but from this sickly state it seems 

 to have recovered, and acquired great vigor; for the next addi- 

 tion which is made to the shell, is equal to its original bulk. It 

 is clean, entire and in perfect preservation, forming a singular 

 contrast with the old shell. 



The place at which shells begin to increase, after the growth 

 has been for some time interrupted may be distinguished by a dif- 

 ference of color in the stripes with which the shell is usually mark- 

 ed. In these places, black or brown stripes exhibit more vivid 

 colors, and sometimes even little different from those on the rest 

 of the superior surface of the shell. The cause of this change is 

 not difficult to trace if we recollect that the secretory organs which 

 prepare the coloring matter, at least in the Helix nemoralis have 

 their origin at some distance from the extremity of the neck, from 

 which we have seen that the first layer of shell which is traced to 

 the extremity itself, should be of a different color from that of the 

 stripes; but as the increase of the animal occasions the stripes to 

 be formed under this first shell, during which it is still very thin, 

 and consequently transparent, it does not prevent the shell pro- 

 duced under it, of a black color, to appear so. But when the 

 animal has ceased to grow for sometime, it then increases the 

 thickness of the shell last formed, so that the shell Avhich is next 

 produced from the coloring matter, when the animal begins to 

 grow, being laid on one part of the old shell much thicker and less 

 transparent, the color of these stripes must appear less bright, and 

 therefore different in those places, from the other parts of the 

 shell. 



In taking a review of what has been said concerning the pro- 

 duction of the colors of shells, it must appear that these rays or 

 colored lines are owing to glands which secrete the coloring fluid, 

 and which are arranged on the anterior edge of the neck, while 



