CONCHOLOGY. 



117 



rontally, even where one valve is not attached to any 

 substance permanently, the lower side is always 

 colourless ; the Spondylus, many Pectens, and some 

 other genera, fully illustrate this fact. Shells inclosed 

 in sponge, or other foreign bodies, are invariably 

 colourless. It must, therefore, be admitted that one 

 lobe of the animal's mantle not receiving the exciting 

 action of light, produces no colouring pigment, and 

 the contrary with regard to the other ; or perhaps the 

 pigment itself only becomes coloured by the action of 

 light on the fluids of the animal's circulation, so that if 

 the shells were removed from the place of their habi- 

 tation, and their position reversed, there would be a 

 consequent change in the coloration of their valves, 

 which is exhibited in some species of the Pleuronecta. 

 It may generally be remarked, that those shells the 

 most exposed to the action of light and heat, are 

 invariably the most brilliantly coloured. The Helices, 

 which are terrestrial animals, in fact, vary the most in 

 colour, and have them of the most vivid tints ; 

 but when they inhabit the umbrageous shades of the 

 forest, they become paler, and less distinctly marked, 

 than the same species living in open situations ; the 

 coloured portions of the shell itself are more intense 

 than those partially concealed of the samfe hue. 



Almost every colour of the rainbow is to be met 

 with in shells, varying in their degree of intensity ; 

 the most common, however, are the brown and fawn ; 

 green is incomparably the most rare, the peculiar 

 markings of these colours depending upon the ana- 

 tomical structure of the animal secreting them, cannot 

 so clearly be understood as we could wish ; indeed, 

 the causes are as yet very incompletely known, but 

 they are infinite in their combinations and variety, 

 and baffle the skill of man to imitate. What can 

 exceed the beautiful pencilling of the Cones and 

 Volutes, or the vivid hue of their colours, and those 

 of many other genera too numerous to be here 

 named. 



It being understood that shells are the protecting 

 coverings of the animals formingand inhabiting them, 

 that they are of various forms, corresponding with 

 the naked body of the animal ; more or less thick, ths 

 fracture either short or lamellar, and constantly at- 

 tached to the body of the animal ; we shall now pro- 

 ceed to explain some of the technical considerations 

 connected with the study of the science of Tcstace- 

 ology, without any reference to its enlarged view, 

 which will be entered upon under Malacology. In 

 the first place, considering these bodies in a general 

 point of view with regard to their structure, the first 

 divisions of shells may be considered between those 

 that are called true or false. A false shell (pseudotcsta*) 

 is that not belonging to a molluscous animal, or rather 

 a shell composed of small polygons, placed side by 

 side, the whole forming a hard, brittle, calcareous 

 envelop, as in the Echini. A true shell is that which 

 is formed by laminae or strata, each one placed beneath 

 the previous one, the most recent and considerable 

 being the last internally deposited, the smallest in 

 substance and the oldest in formation being the exter- 

 nal ; this applies to every species of shell, whatever 

 may be its form, or the number of pieces composing it. 

 A general study of that form occasions another divi- 

 sion between such as are tubular or not, but little 

 explanation is necessary to point these out, as every 

 one will easily recognise them from such as are spi- 

 rally formed. The nontubular shells are divided into 

 such as consist of one piece these are the univalves ; 



the sub-bivalves are those consisting of a principal and 

 an accessory part ; bivalves are all those consisting of 

 no more than two pieces ; tubivalves are such shells 

 as have two principal pieces, with an accessory one ; 

 and the multivalves are such as have a greater number 

 of portions or pieces. A valve is therefore a calcare- 

 ous portion of shell, varying greatly in point of form, 

 either concealed within the animal's skin, or protect- 

 ing a greater or smaller portion of the animal s body. 

 Multivalve shells are of three descriptions ; those that 

 are composed of transverse pieces, as in the Chiton ; 

 those that are formed of five valves, or more, symme- 

 trically arranged to the right or to the left hand, some- 

 times placed in the form of scales united together by 

 means of the animal's skin ; these form a division 

 established by De Montfort, under the title of dissi- 

 valvcs, as occurs in the genus Anatifera; ; and, lastly, 

 those shells in which the pieces or valves are placed 

 in a circular manner, as in the genus Halamts and its 

 congeners, which form Lamarck's sub-coronals. Tubi- 

 valve shells are those composed of two principal 

 valves, as in bivalves, properly so called, but sur- 

 rounded or enveloped by another piece in the form of 

 a tube, which, nevertheless, cannot be considered as 

 another valve ; this is observed in the genera 

 Teredo, Fistulana, &c. Bivalve shells are such as 

 their name indicates, formed of two pieces, these two 

 pieces notwithstanding being only one cover or dwell- 

 ing for the animal inhabiting it; they are nearly in 

 every instance placed on the sides of the animal, and 

 connected with each other, inasmuch as the functions 

 of the animal are double, and those which apply to 

 one valve apply equally to the other. Where the valves 

 are not entirely similar in their configuration much 

 error has been occasioned ; and Linnaeus considered 

 the Lingu/a an univalve, having seen only one valve, in 

 the genera Orlncula, Cliama, and some others ; the 

 dissimilarity of the valves, were they not well known 

 to naturalists, might lead to similar false conclusions. 

 Sub-bivalve shells are those in which, besides a piece 

 analogous to that which constitutes the univalve 

 shells, there is a second more or less perfect cal- 

 careous or otherwise*, closing more or le*s completely 

 the aperture of the shell, which piece is called the 

 operculum. 



Univalve shells merely present one constituted 

 piece, no matter what its form, or how differently it 

 may be modified in the different species ; it altogether 

 forms an asylum for the animal in cases of repose or 

 danger, or, as we have already stated, protects such 

 organs of delicate sensibility at are the most exposed 

 to accident. An univalve shell may be completely 

 tubular, or entirely concealed in the animal's skin. 

 Having drawn these distinctions, we will proceed to 

 examine each of these species of shells, commencing 

 with the most simple, and going on to the most com- 

 plicated structure ; but, in the first instance, we will 

 define the terms derived from the analogous habits of 

 each, which consequently apply equally to univalves 

 as well as to bivalves. Shells, no matter what num- 

 ber of pieces compose them, must be considered as 

 alike in various circumstances common to till, which 

 we will rapidly examine. From the situation in which 

 Molluscs are found, they have been distinguished into 

 terrestrial ; rluviatile, or those inhabiting fresh water ; 

 and marine, or those dwelling in salt water. 



Terrestrial are such as are found on land, and are 

 so organised as not to be able to exist in a tluid ; they 

 breathe air only. 



