CONCHOLOGY. 



123 



most bivalve shells ; on the contrary, the valves are 

 irregular when they assume the shape of the substance 

 to which they adhere, such as the oysters Anomia, 

 &c. This occurs with both valves in some instances, as 

 the form of the upper is necessarily modified by that 

 of the lower valve : whether the valves are thick or 

 thin, is so obvious, that no particular term is necessary. 

 The valves are plaited (uiflexa), when they form an 

 angle or fold projecting a*, their inferior part, as in the 

 Tellens. Each valve, regular or otherwise, may 

 really be compared to an univalve shell, in general 

 very flat and covering the animal, but instead of being 

 placed on the back of the animal is situated at its 

 $ides ; there must, therefore, exist a summit and a 

 base, an external convex, and a concave internal 

 surface. The summit of a bivalve shell is termed the 

 beak, or tip (apex), and it is from that point the growth 

 of the shell commences. Considering its general 

 position, always bearing in mind the animal which it 

 cover*, it is oral or buccal (oralis), when it is at the 

 anterior extremity of the valve, which is rarely the 

 case ; an exam pie, however,is exhibited in thespondylus, 

 oyster, &c., and this is commonly called the spur or 

 heel. It is dorsal (dorsalis), when it corresponds with 

 the back of the animal or the superior margin of the 

 shell, which is ordinarily the case with shells in general ; 

 in this case it may be antero-dorsal, when placed more 

 forward than backward in the length of the valve ; 

 mediii-dorsaL, when in the middle ; and postcro-dorsal, 

 when mure backward than forward ; anal or posterior 

 when situated at the opposite extremity to the mouth, 

 as in the Terebratula, Lingula, &c. 



It is also from the relative position of the summit 

 of the bivalve shells, that the characters are indicated 

 by equilateral, sub-equilateral, or in-equilateral. A 

 valve is said to be equilateral when the cephalic sum- 

 mit is exactly in the middle of the side on which itis 

 placed, so that a perpendicular line drawn through 

 would sub-divide it in equal portions, as is seen in the 

 Pfctcns. Sub-equilateral, when there is only a slight 

 difference in its position, either backward or forward, 

 In-equilateral, when the difference between the two 

 sides is more considerable, and consequently the 

 summit becomes either antero-dorsal or postero-dorsal. 

 The direction of the summit offers also some character 

 designated by particular terms ; it is the most frequently 

 a little curved or inclined forward, but it is also some- 

 times entirely vertical, or in the direction of the 

 diameter of which it forms one extremity, and it i 

 more rarely inclined backward ; and in some species 

 the summit has a tendency to assume a spiral form, as 

 in univalves. It is variously named in scientific terms 

 according to its being ear-shaped, spiral, recurved 

 entire, callous, &c., for which, as before, we refer to 

 the treatises on this subject. The external face of the 

 valves offers several characters important to know, it 

 is either convex or flat, which needs no explanation 

 it is full (plena), as in the Tridacna ; bordered (mar- 

 ginata), when furnished with a projecting rim ; toothec 

 or serrated (dentata), when the circumference is 

 furnished with what may be here termed teeth, as in 

 the tridacna, cordiform, crescent-shaped, lanceolate 

 oval, oblong, superficial, &c., according to its resem- 

 blance to a heart, as in some of the species of Venus, 

 The internal surface of the valves presents also many 

 characters deserving attentive consideration, but not 

 easily described without illustrations, that would here 

 occupy too great a space. The valves are right hanc 

 or left, according to the position of the animal as we 



lave already placed it ; they are equivalve,when exact- 

 y corresponding in size, depth, and general form, as 

 exists in most of the bivalves ; sub-equivalve, when 

 he difference between the two valves is not very con- 

 siderable, as in certain species ofPcctens; in-equivalve, 

 when there is a very visible difference, either in the 

 ibrm, size, or other particulars. Linnaeus has named 

 ;he smallest and flattest valve the operculated one, as 

 m the Gryphcea; but, for the reasons we have stated, 

 it is mis-applied. The valves of bivalve shells, placed 

 in opposition to each other, often touch at every 

 point of their circumference, in which case they are 

 termed closed (clausa), when not thus everywhere 

 united, they are said to gape (hians). In the first 

 instance, the closing of the valves may be simple by 

 approximation, or they may be shut by the insertion 

 of the opposite notches, grooves, teeth, &c., with which 

 many are furnished. In bivalves having a partial 

 opening in the circumference of the valves, more or 

 less considerable, it is, for the most part, only posterior, 

 or both anterior and posterior to a certain extent, as 

 in the Solens, the Pliolas, and many other species ; 

 sometimes it is inferior, and more or less anterior or 

 medial, as in the Arks, Muscles, &c., and it is some- 

 times anterior, and superior, as in the young examples 

 of the Tridacna. Another point ot view, perhaps 

 more important than most of those which we have 

 described, is the peculiar mode in which the valves of 

 shells are united to each other ; these are three in 

 number, one belongs exclusively to the animal, 

 depending upon the use of certain muscles or bundles 

 of muscular and elastic fibres, which are carried trans- 

 versely from one valve to the other ; these leave an 

 impression on the internal surface of the valve, the 

 study of which has been pointed out as important in 

 deciding the genus of a mollusc, without any previous 

 knowledge of the animal. The second mode of 

 union also belongs properly to the animal alone, and 

 although much less than the first, it leaves also its 

 indices, or traces, very perceptible in the excavations 

 of different forms in which it is attached, this is 

 named the ligament (ligamentum) ; it consists of a 

 mass, more or less considerable, of horny fibres, 

 elastic, and covered with an epidermis, carried from 

 side to side of the valves. Some bivalve shells are 

 met with entirely without a ligament, properly so 

 called, as in the Orbicula, Pholas, and others, in which 

 it is not distinct from the general epidermis, as in the 

 Pinna, but most frequently bivalves possess a liga- 

 ment ; this is simple (simplex) when there is only one, 

 as in the Venus ; and the greater number of bivalves, 

 double (duplex), when there are two ligamental pro- 

 cesses, the one posterior, as in certain Tellens, w hence 

 they are called amphidesma, or when there exists one 

 external, the other internal, as in the Mactra. Mul- 

 tiple (multiplex), when there is a series more or less 

 considerable, as in the Perna, and, perhaps, even in 

 an inverse position, in the Arks. The position of the 

 ligament, with regard to the summits, is thus defined : 

 anterior, when placed in front of them as in the 

 Donax; medial, when immediately beneath the apices ; 

 posterior, which is the most usual, when behind the 

 summit ; antero-posterior, when both backward and 

 forward, occupying an extended space, as in the Arks 

 and their congeners. The position of the ligament, 

 as it is externally visible or not, distinguishes it into 

 external (externum), when visible, which occurs with 

 most bivalves ; deep (profundum), when it is so deeply 

 placed, that it is scarcely perceptible on the outside; 



