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lowing the trace of the wreath. The neck of the growing fnail is the part which principally forms the Jhell ; this is al- ways naked, and is always making itfelf a covering, which, while the neck grows beyond it, fcrves for the next part. Now if we fuppofe this neck to be yellow, with one black fpot in a particular part ; or rather, that it all over exfudates a yel- low liquor, except in fome one fpot, where it exfudates a black one ; then it neceffarily follows, that as the growth of the animal is performed in a fpiral line, and the jhell made accordingly, while the yellow matter makes a wreath of yellow jhell furrounding the former, the black part muft alfo be drawn in the fame fpiral form, as we fee a fpiral line of that colour in the wreath. And accordingly, if there be more points than one of this kind, then there muft be more rays or lines alfo in the jhell ; and if thefe points are of dif- ferent colours, then the rays formed by them will be alfo of different colours ; neither are thefe black points, or fpots, to be treated as a mere conjecture ; they in reality ever ap- pear upon the animal ; and if a growing fnail be nicely in- fpefied, they are always found placed juft even with the black lines, or rays, which are carrying round the Jliell. The part of the body of the fnail which immediately follows the neck, and fills up that part of the Jhell it has juit formed, difcharges neither a yellow nor a black liquor, but only a thin aqueous whitifh humour ; this condenfing on the infide of the new formed jhell coats it over, and forms its inner cover- ing, which therefore is always whitifh, and has no varieties of colour.
It will be very eafy for every naturalift to extend what is here related of the common garden fnail to all the other animals which are covered in the fame manner with jhelk ; and the differences of colour in the matter tranfpired through the neck of the growing animal, with the different number and difpofition of the variegating points or dots on it, may give fufficient room to account for all the beautiful variety of colours, and all the variegations in the raoft elegant {ea-Jhells.
The other varieties of them may alfo be accounted for upon the fame principle ; thus, fuppofe the growing fifh of one kind has a number of eminencies on its body, there cannot fail to be prominences of the fame kind in the jhell that is formed by the regular tranfpiration of its juices ; and thefe will go on regularly, enlarging to the end of the jhell, becaufe the prominences which form them will grow larger in the animal as it grows. If there be any certain feafon of the year when theyiW/ceafes to grow, that is when the animal ceafes to enlarge its body, fuppofe this to be the fummer or the winter, or whatever particular feafon ; in this cafe, the traces of the different times of the creature's begin- ning to grow again, will not fail to be marked upon"the jhell ; and the age of the Jhell might, if thefe times were known, be perhaps, in many cafes, as well determined by thefe feveral junctions of new to the old matter, as trees are known by their annual concentric circles. Thus we fee that thefe tender animals are themfelves the architefls of thefe their elegant habitations, which are fo beautifully regular, merely becaufe they grow as it were with the body of that architect:, whofe different age and ftate of life, if perfeflly underftood, they would be found elegantly to denote Mem. Acad. Par. 1 709.
The durable hardnefs and excellent polifh of many of the Jhells of fea-fifhes, and that even in the thinneft and tendered kinds, is a thing very amazing. In the ruins of the abbey of St. Edmundfbury, which is built of a kind of ftone com- pofed of grit or fand, interfperfed with an infinite variety of very minute jhells, principally of the fmall fmooth cockle- kind, Mr. Collinfon obferved, that the ftone having fuf- fered greatly by the injuries of the air, the grit or fand of which it was compofed had difunited and mouldered away while the fmalleft of thefe Jhells flood the fame injurious ac- cidents unaltered, and were found ftanding forth in the ut- moft perfeaion, and with all their beautiful natural polifh ■ yet thefe are as thin and tender a Jhell as almoft any of the fame fize that we know of.
It is poffible, indeed, that thefe jhells might have been al- tered in their nature, by the infinuation of ftony matter into their pores, as is a common cafe in Jhells lodged in ftone • but thofe, here mentioned, did not feem to have undergone any fuch alteration ; and it is very certain, that many of the like thin and tender Jhells are found preferved through a long fenes of ages, in places where they have been in the way of no fuch alterations from the infinuation of ftony matter into their pores, and out of which they are taken fair and beau- tiful and with all their natural polifh, though not at all hardened in the time. Of this kind are the Jhells of the ten- der buccinums, and other thin kinds, found buried in earth in the ftrata of marl, clay, or loam, and even in chalk As hard as thefe Jhells naturally are, they do not, however, prefervethe fifh, which inhabit them, from becoming the
to brrL°t erS ' W , ' eem '° haVC n ° ° r S ans with ^hich
bellv „ro 'Vu C T m °" *° fee the foal - fifll With its
V whT nent f d ha I d ' J aS if We " fi " ed with f Pa™> »"° yet when opened to find no fpawn at all ; but the oro-
m ™ Vo^lf " 0CC = fl0ned ^ftom-h and guts
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being filled I with Jhell-m, which the creature had fwallowed whole, as food, without having any power of breaking or deftroying them. A fmall fpecies of peaunculus, or cockle is the fifh that the foal ufually feeds upon ; and when the guts are taken out of this fifh, they often referable fo many ltrings of necklaces, from the continued feries of thefe A/i which appear prominent in them. When thefe Jhells are ta- ken out, many of them are ufually found in part diffolved, and many entire and unaltered.
Shell-iVh are well known to be the food of fome other fifties of the larger kinds, particularly the fea-porcupine, and a kind or the wray-fifh, are known principally to feed upon them ; but thefe fifh are provided by nature, with a fuitable apparatus fitted for the grinding, or reducing them into a ftate more proper for digeftion ; their jaws being furnifhed with bony fubftances, extending to the palate and under part of the mouth, which arc capable of reducing much ftronger Jhells than thefe into an abfolutc pulp. The foal-fifh has no apparatus of this kind, and neither its jaws norftomach are furnifhed with any thing capable of wearing them to pieces by attrition ; but what appears to be the cafe, is, that na- ture has furnifhed this creature with a menftruum in the body capable of digefting them. Phil. Tranf. N° 477. p. 30 The crab, like the lobfter, bV. cafts its Jhell once every year. Agamft this extraordinary event, the creature always choofes a clofe and well fecured retreat in the cavities of rocks or under great ftones ; there they creep in, and wait till all the parts are by degrees difengaged, which is effeSed by withdrawing their legs from their old Jhells, and leaving them and the upper part of their bodyyM behind. In this naked ftate they make a very ftrange appearance, being a mere ill-fhapen lump of a ffefhy matter, covered with a fort of jelly ; this by degrees hardens into a jhell, a degree larger than the old onecaftoff. Phil.Tranf. N° 47 8. Seft. 14.
"'Wing of Shells. See the article Polishing.
Foffile Shells. The number and variety of ka-Jhel/s which are found far from feas buried at great depths in the earth, and often immerfed in the hardeft ftones, is an obiea of great wonder.
Of thefe fome are found remaining almoft entirely in their native ftate, but others are varioufly altered by bein» im- pregnated with particles of ftone, and of other foffiles ; in the place of others there is found mere ftone, or fpar, or fome other native mineral body, expreffing all their linea- ments in the grcateft nicety, as having been formed wholly from them, the Jhell having been firft depofitcd in fome fohd matrix, and thence diffolved by very flow degrees, and this matter left in its place, on the cavities of ftone and other fohd fubftances, out of which Jhells had been diffolved and wafhed away, being afterwards filled up lefs (lowly with rfa d,fferent; fubftances, whether fpar or whatever elfe : thefe fubftances, fo filling the cavities, can neceffarily be of no other form than that of the Jhell, to the abfence of which the ca- vity was owing, though all the nicer lineaments may not be fo exaffly exprefied. Befide thefe, we have alfo in many places mafles of ftone formed within various Jhells; and thefe having been received into the cavities of the Jhells while they were perfeaiy fluid, and having therefore nicely filled all their cavities, muft retain the perfea figures of the internal part of the Jhell, when the Jhell itfelf fhould be worn away, or perifhed from their outfide. The various fpecies we find of thefe are in many genera, as numerous as the known re- cent ones ; and as we have in our own ifland not only the Jhells of our own ftiores, but thofe of many other very diftant ones, fo we have alfo many fpecies, and thofe in great num- bers, which are in their recent ftate, the inhabitants of other yet unknown or unfearched feas and fhores. The cockles, mufcles, oifters, and the other common bivalves of our own feas are very abundant : but we have alfo an amazing num- ber of the nautilus kind, particularly of the nautilus gra:- corum, which though a. Jhell not found living in our own, or any neighbouring feas, yet is found buried in all our clay-pits about London and elfewhere ; and the moft frequent of all foffile Jhells, in fome of our counties, are the concha; anomiae, which yet we know not of in any part of the world in their recent ftate. Of this fort alfo are the cornua ammonis and the gryphitje, with feveral of the echinitae and others.
The exaS fimilitude of the known jhells recent and foffile in their feveral kinds, will by no means fuffer us to believe, that thefe, though not yet known to us in their living ftate, are as fome have idly thought, a fort of lufus naturae. It is certain, that of the many known fhores, very few, not even thofe of our own ifland, have been yet carefully fearched for the Jhell-fiSh that inhabit them ; and as we fee in the nautilus graecorum an inftance of Jhells being brought from very diftant parts of the world to be buried here, we cannot wonder that yet unknown fhores, or the unknown bottoms of deep feas, fhould have furnifhed us with many unknown Jhell-R(h, which may have been brought with the reft ; whe- ther that were at the time of the general deluge, or the effeft of any other cataftrophe of a like kind, or by whatever other means, to be left in the yet unhardened matter of our ftony and clayey ftrata. Hilts Hift. of Foil', p. 616.
2L11 Tha
