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CONCHOLOGY. 



Such only are known in the division of univalves 

 and sub-bivalves, as it is quite impossible that bi- 

 valves or multivalves, which are tenanted by aquatic 

 animals, breathing water, should be found on land. 



Terrestrial shells are generally very thin ; their 

 exterior surface usually smooth, presenting no other 

 inequalities than the striae, indicating the different 

 stages of growth, sometimes with pilifonn prolonga- 

 tions of the epidermis, but never with spines, or other 

 protuberances properly so called ; their internal or 

 external surface is never pearly ; their aperture is 

 always entire, particularly in the adult state, with the 

 right side of the lip thickened into a ridge more 

 or less turned outwards. 



Fluviatile shells comprehend all those that live in 

 fresh water, whether stagnant or current ; some of 

 them are found in the three first divisions of shells, 

 viz. the univalves, sub-bivalves, and bivalves, but 

 none are yet known in the multivalves ; the number 

 of fluviatile shells is considerable compared with the 

 others, and is confined to a very few families. So 

 amongst univalves, the greater number known have 

 the aperture entire, either operculated or not, but 

 above all in the last group. One or two genera, at 

 the most, have it grooved, though very slightly, as in 

 the Melanopsis, and there is perhaps only one genus in 

 which it is patelliform, like a Limpet. The bivalves 

 of this section only belong to three or four families ; 

 the only ones known are those of the Mytilus, Sub- 

 mytilus, Chama, &c., and even then it is often a single 

 genus, or a solitary species. Fresh water univalves 

 and sub-bivalves arera'general thin, since they belong 

 to swimming animals ; their external surface in gene- 

 ral finely striated, without bands or varices, but some- 

 times with spines ; and when there is an epidermis, 

 it is always very thin. One of their general characters 

 is that of having their aperture entire, but the Mela- 

 nopsis forms an exception to the rule, not only in that 

 respect, but as being often of a considerable thick- 

 ness : thus the distinction between fresh water uni- 

 valves and sub-bivalves is less easy than in the 

 terrestrial. Fluviatile bivalves are nearly in the same 

 case ; it has, nevertheless, been remarked, that whe- 

 ther the substance be thick or thin, they have nearly 

 always a rather thick epidermis, the valves shut 

 completely, they are more or less pearly within, and 

 the summits decorticated in most species. 



Marine shells may be said to exist of all families, 

 except those of the snail, and other of their congeners, 

 and of the first section of the Submytilus among the 

 bivalves. Their characters are opposed to those of 

 the two first sections. Thus the univalves, and even 

 the sub-bivalves, are in general much thicker, and 

 more frequently marked with ridges, varices, and 

 spines. Their aperture also differs from the preceding, 

 by being very frequently channelled, and bordered on 

 the right side by a simple or complex ridge. They 

 are sometimes pearly on the inside ; the epidermis is 

 often scaly, piliform, and, generally speaking, totally 

 different from either the fluviatile or terrestrial 

 species. Marine bivalves are in general more rough 

 on their exterior, with more strongly defined striae, 

 grooves, and rays ; their epidermis is less smooth, even 

 thinner, and usually of a very different form from that 

 of the fluviatile shells. With regard to the parts of 

 rivers, lakes, and seas, in which molluscous animals 

 live, they are sub-divided into littoral and pelagic 

 shells ; the littoral are those, either univalves, 

 sub-bivalves, bivalves, or multivalves, which inhabit 



more or less constantly the sides or banks of rivers, 

 lakes and seas ; the pelagic are, on the contrary, 

 those molluscs always inhabiting the deepest parts of 

 the sea, in consequence of which they are the most 

 difficult to come at, and but generally only known 

 through accidental circumstances. There is not, 

 however, any permanent distinguishing character 

 for these shells, so as to enable the naturalist to 

 decide, by an examination of the shell only, to what 

 habitat it belongs. A moderate degree of experience 

 will, nevertheless, give certain rules going far to 

 establish the fact, without any knowledge of the 

 animal. There is not, either, any indications on the 

 shells themselves to point out such of them as are 

 found embedded (in some instances it is difficult to 

 say how) in wood, stone, &c. As a general defini- 

 tion, these are called (Terebrantes) borers or piercers, 

 and distinguished from each other, as Petricolce, or 

 Lithodom<, when found in calcareous substances, of 

 a more or less hard texture, as with the Pholodes, 

 &c. ; Xylodomce, or Lignivorce, when they have 

 established their habitations in wood, as is the case 

 with the Teredo: Arenicoke, when they inhabit the 

 sand ; Lutricol<, when buried in the mud of rivers, 

 ponds, &c. -An observation may, however, be here 

 made with respect to such of the penetrating molluscs 

 which inhabit hard bodies. They are only to be met 

 with in some families of bivalves, and a tolerably 

 uniform indication may be discovered in the larger 

 and more rounded form of their anterior extremity. 

 It is also pretty easy to distinguish, by certain marks, 

 such molluscs as are more or less fixed in their local 

 habits. These shells may be divided, in consequence, 

 into those being (Natantcs) floating, that is, inhabited 

 by swimming animals. They are then wisely con- 

 structed of a very thin light substance, which is 

 beautifully exemplified in the Argonauta, Janthina, 

 Sulla, and others. 



The Libcrce, or free shells, are such as are more 

 solidly constructed, and presenting no sign of ad- 

 herence, or power of attaching themselves to sub- 

 marine or other bodies. This is the case with the 

 greater proportion of shells. 



Adluzrentes, or adhering, when they are altogether 

 immovable, or only possessing the power of moving 

 in a very limited space. To the extent of their 

 attachment, these are denominated 



FixtE, or fixed, when their attachment is by means 

 of the adhesion of the substance of the shell itself, as 

 is the case in univalves with the 



Hipponix ; in the bivalves with the Spondylus, 

 Oyster, Chamae ; and in the Balani among the 

 multivalves. 



Radicate, rooted, when the adhesion is effected by 

 means of a tendinous part, as is the case with the 

 Terebratula and Lingula in bivalve shells ; the 

 Atiatifce in multivalves. This Linnxus improperly 

 designated intestinum, an error perpetuated by Bru- 

 gniere, who translates it intestine. This adhering 

 portion is often marked in the shell, but it is some- 

 times indicated by a hole or groove in one of the 

 valves ; and lastly, molluscs are called Obtecta, or 

 hidden, when they are concealed at a certain depth 

 in a distinct tube, to which they adhere, or remain 

 loose, enveloping the animal altogether, as in the 

 genus Fistulana and its congeners. We will here 

 describe the position of the shell as viewed by modern 

 naturalists, in describing the various names ascribed 

 to its differen^ parts. In this a well-marked and 



