CONCH O LOGY. 



113 



and food, continues for a definite period ; with 

 regard to the animal, whose increase of bulk requires 



increased accommodation in its shell, as would 

 appear particularly in the genera we have already 

 named, and in all the other genera whose varices, 

 whether spinous or foliaceous, seem placed at stated 

 intervals, some being found at the half turn of the 

 whole circumference of the shell, others at a quarter, 

 some one third, and some perpendicular to the base ; 

 others at the sides of the shell, giving it a flattened 

 or ventricose appearance, according to the position 

 of these previous terminations, which, it must be 

 again observed, are formed of the vitreous, and not 

 lamellar substance. In bivalve shells, also, the period 

 of growth may be traced by their striae, and they 

 seem also to have a limited extent of increase, as, 

 generally speaking, the various genera attain a tole- 

 rably uniform size. Their age is, however, more 

 strongly marked by the thickness of the valves, and 

 the internal muscular impressions, than any other 

 guide. The valves of shells having a continual 

 tendency to fly open, in consequence of the elastic 

 ligament situated at the side of the hinge, which 

 operates as a muscle, it was necessary that the 

 animal inclosed should have the power of closing 

 them at pleasure. There are, therefore, according to 

 the different genera, alwa3 r s one or two muscles fitted 

 for this purpose. In the oyster there is only one 

 muscle of this kind, situated near the centre of the 

 shell, behind the liver, and in the middle of the cloak. 

 It is inserted into both valves, and by its contraction 

 brings them together with an astonishing force, equal 

 to the pressure of many hundred pounds weight. 

 The same mechanism prevails in the 1'er-na, Avicula, 

 and Spondylus. In young shells these muscles leave 

 a fainter impression than in old ones, in which the 

 increased substance of the valves render them deeper. 

 They are, as it were, always advancing more towards 

 the edge of the valves, not that these muscles are 

 ever displaced entirely, but new portions of them are 

 formed as the animal finds it necessary to occupy a 

 convenient position in its dwelling ; the posterior 

 parts of the muscle become inactive, and die away, 

 whilst others replace them on the anterior part, and 

 so on during the life of the animal. These muscular 

 impressions also afford an excellent guide to the 

 genus of animal to which a shell belongs, where no 

 other opportunity presents itself of judging. The 

 difference which exists ia the size of shells of the 

 same species, particularly in such as have not the 

 strongly marked lip, or other final termination, is 

 very remarkable. It does not appear easily accounted 

 for, but a similar anomaly exists in every branch of 

 natural history ; and it may therefore be attributed 

 chiefly to the strength and constitution of the ani- 

 ;mal, being always proportioned to the favourable 

 or unfavourable situation of its habitat, and the 

 goodness of its nourishment in peculiar situations. 

 Another extraordinary occurrence is, the elongation 

 of the spires of some univalve shells, in which the 

 whorls do not touch each other, but appear drawn 

 out like a coiled spring. The Scalaria and Helix 

 are those in which that lusus has been observed, 

 though it is very rare ; but the greatest monstrosity 

 of all, the most inexplicable freak of nature, is dis- 

 played in such shells as are sinister or heterostrophe, 

 that is, having their whorls turned to the left instead 

 of the right. This can only be accounted for by the 

 viscera of the animal presenting a change in their 



NAT, HIST. VOL. II. 



position, which of course is followed in the formation 

 of the shell, this is to be understood, but the former 

 baffles inquiry. Such a phenomenon might therefore 

 occur in every species of mollusc, and does so in 

 numerous instances ; but some of the genera, parti- 

 cularly of terrestrial shells, are more commonly left- 

 handed than on the contrary. These, however, do 

 not, in the opinion of the author of this article, con- 

 stitute anything beyond a variety or sport of nature, 

 though constantly occurring in some genera. 



The probable number of shells in a recent state, 

 can only be guessed at ; as, in addition to the species 

 already known,, naturalists and travellers are con- 

 tinually increasing the amount by new researches ; 

 but judging, which may fairly be done, from the 

 immense beds of the vestiges of former testaceous 

 animals, we may conclude, that living examples exist 

 in countless myriads. 



The mode of production is various ; some of these 

 creatures are viviparous, or bring forth their young 

 alive, as is the case with most of those which inhabit 

 bivalve shells, multivalves, and even some of the uni- 

 valves, while others, which form by far the greater 

 portion of such as are known, are oviparous, or 

 produce their young from eggs. The animals con- 

 structing shells, do not, generally speaking, exhibit 

 any known affection towards their progeny, though 

 many possess that wisdom termed instinct, in deposit- 

 ing them not only in secure places, but especially 

 where food, the first want of nature, will be amply 

 supplied, without much trouble to obtain it. Adarison 

 mentions that the female of the Voluta gondola of the 

 Linnaean school, collects its young for some time 

 together in the fold of its foot, 'and the female of the 

 oviparous Paludina also carries its young some days 

 on its shell. Species of the Succinum, Purpura, and 

 probably many other genera, whose eggs are pe- 

 dunculated, or having a stem, are. attached, at their 

 exclusion, to the neighbouring substances. The 

 Janthina, which is always found floating, surrounds its 

 shell with eggs ; the snail and others of its congeners 

 conceal their eggs in the hollow of trees, beneath stones, 

 in damp places protected from the sun ; the Ocythoe, 

 of the Argonauta, appears to place its eggs at the 

 bottom of the shell it inhabits ; muscles attach their 

 youna: by means of their channelled foot ; and Balani 

 do ihe same with the long tube which terminates their 

 oviduct. Most other molluscs lay their eggs as chance 

 directs ; and it is possible, from the viscous matter that 

 surrounds them, they are enabled to adhere to marine 

 substances, where, at the period of their development, 

 they find themselves the most advantageously placed 

 according to their future wants and habits. By far 

 the greater number of eggs produced by molluscous 

 animals are mucous ; some are horny, and some are 

 covered with a calcareous coating, resembling those 

 of birds, as is the case with one or more species of 

 Achatina and Bulinus. In one fact all naturalists 

 agree, that whatsoever the mode of production, 

 whether from an egg or otherwise, the shell is formed 

 on the body of the young animal proportioned to its 

 then bulk ; the increase this shortly requires, and the 

 consequent necessity of enlarging its shelly dwelling, 

 we will now explain. For this purpose the common 

 snail shall be our preceptor ; its example holds good 

 with all shells, being formed of spiral convolutions, 

 whether they are aquatic or terrestrial. The princi- 

 pal use of shells is, doubtless, to cover the animal 

 entirely when in want of such a protection, or merely 

 H 



