GASTEROPODA. 



383 



calcareous varnish, which being laid over the 

 exterior of the old shell completely conceals 

 the original markings ; these, however, may be 

 again exposed on removing with a file the outer 

 crust: a line, which is generally very distinctly 

 seen running longitudinally along the back of 

 the shell, indicates the spot where the edges of 

 the two alae of the mantle met during the com- 

 pletion of this singular process. Such shells 

 are therefore remarkable from the circumstance 

 of having their thickness increased by additions 

 to the outer as well as to the internal surface. 



In terrestrial shells it is only when they have 

 arrived at their full growth that a rim or margin 

 is formed around the aperture, which serves to 

 strengthen the whole fabric; but in marine 

 shells, which attain to much larger dimensions, 

 the growth is effected at distinct periods, each 

 of which is indicated by a well-defined margin, 

 and these ridges remaining permanent, the suc- 

 cessive stages of increase may be readily seen. 

 At each suspension of development, it is not 

 unusual to find spines or fringes, sometimes 

 differently coloured from the rest of the shell, 

 and not unfrequently of considerable length. 

 In Jig. 180, which represents the shell of Murex 



Fig. 180. 



cornutus, the nature and arrangement of such 

 spines is well exemplified. They are all formed 

 by the margin of the mantle which shoots out 

 into long fringes, encrusting themselves with a 

 shelly covering ; each spine therefore is at first 

 hollow, and if in many species they are found 

 solid, it is because the original cavity has been 

 gradually filled up by the deposition of earthy 

 matter within it. The syphon with which many 

 Conchiferous Gasteropoda are provided is pro- 

 duced in precisely the same manner, and its 

 identity in form with the other spines covering 

 the surface of the shell is in the annexed figure 

 sufficiently obvious. In many species, as in 

 the beautiful Turbo scalaris, (Jig- 181,) the 

 epocha of growth are only indicated by ridges 

 surrounding the shell at regular intervals, each 

 of which originally terminated a fresh augmen- 

 tation of its size. It is difficult to imagine by 

 what influence these creatures are induced to 

 enlarge their habitations at such regular inter- 

 vals, terminating each operation by a similar 

 margin ; some authors imagine that each time 

 the creature emerges from its abode a fresh 

 addition is made ; others that it is dependent 

 upon the temperature or state of the seasons, 

 but without sufficient grounds for either of these 

 assertions ; it seems more probable therefore 

 that the growth of the body gradually rendering 

 the former dimensions of the shell incommo- 



dious from time to Fig. 181. 



time renders these pe- 

 riodical enlargements 

 necessary. 



Although shells 

 are evidently inorga- 

 nic and extra-vascu- 

 lar structures, it is 

 now universally con- 

 ceded that their in- 

 habitants have the 

 power of removing 

 portions which may 

 obstruct their growth, 

 or needlessly infringe 

 upon the limits of 

 their abode. In the 

 Murices we have in- 

 disputable evidence 

 of this fact in the 

 removal of such spines as would interfere 

 with the revolutions of the shell around the 

 columella, and in Conus and similar genera 

 a like faculty enables the animals to thin 

 the walls which bound the inner whirls 

 when their original thickness is rendered un- 

 necessary by the accession of new turns. Such 

 a solvent power indeed is not only exer- 

 cised upon their own habitations, but many 

 Gasteropods are able gradually to bore holes in 

 other shells, or perforate the rocks upon 

 which they reside to a considerable depth. 

 The mode in which this is effected is, however, 

 still a mystery ; some authors ascribe it to a 

 power of absorbing their shells, an expression 

 the vagueness of which is sufficiently evident ; 

 others ascribe it to some acid secretion at the 

 disposal of the animal ; yet although this ex- 

 planation is certainly plausible, when we reflect 

 that the very structure which secretes this sup- 

 posed acid is itself the matrix of such abundant 

 alkaline products, it is not easy to imagine how 

 the same structure can at the same time furnish 

 such opposite materials. 



As we should expect from the mode of its 

 growth, the shell throughout all the Conchi- 

 ferous class is composed of earthy matter, 

 cemented together by an animal substance 

 easily separable by the action of acids. In the 

 porcellaneous shells the animal matter exists in 

 much less quantity than in those of a fibrous 

 texture ; in the former, indeed, Mr. Hatchett 

 found that when the carbonate of lime, of 

 which the earthy portion is almost entirely 

 formed, is dissolved even by very feeble acids, 

 little or no vestige of any membranous struc- 

 ture could be perceived, nor indeed could any 

 be detected, but by the small portion of animal 

 coal which was formed when these shells had 

 been exposed for a short time to a low red 

 heat ; in others however, as the Patellae, a sub- 

 stance was left untouched by the acids which 

 had the appearance of a yellowish transparent 

 jelly, by means of which the earthy matter 

 had been, as it were, cemented together. 



On examining minutely the mechanical ar- 

 rangement of the layers of which these shells 

 are composed, it is found to vary in different 

 kinds, and from this circumstance the fossil 



