CONCHOLOGY. 



appears on one side of the beak ; this 

 last-mentioned character is very obvious 

 in charma gigas. 



One of the first circumstances to be con- 

 sidered is, which part of a bivalve shell 

 ought, to be deemed the base, because, 

 when this is determined, every other part 

 will fall progressively in their relative or- 

 der under our observation. We name that 

 part of the margin or limb which is situat- 

 ed in a direct line opposite the beak, the 

 base of the shell. Linnaeus, in order to 

 establish the characters, and afford some 

 apparent re'ason at least for the application 

 of the terms he bestows on the different 

 parts of bivalves, reverses this position* of 

 the shell, and describes the beaks as the 

 base of the shell. But the fact is, the na- 

 tural position of the shell is in immediate 

 contradiction to his axiom, for the beaks 

 are always uppermost, being either imme- 

 diately vertical, or with a slight inclina- 

 tion obliquely, when the animal moves 

 along with its testaceous covering on the 

 back. A solitary example will perhaps 

 occur occasionally, in which the beaks 

 may be considerably inclined when the 

 animal crawls, but none, we believe, are 

 known which open the valves upwards, 

 and proceed with the beaks under the 

 body. The beaks, if only for this reason, 

 are to be considered as the summit, and 

 the margin opposite as the base. Many 

 of the bivalves are destitute of the loco- 

 motive power, or at least do not possess it 

 in any material degree. 



Summit, a word applied in a general 

 manner to the top or most elevated part 

 of the two protuberances observable in 

 the greater number of bivalves. Da Costa 

 calls that part of the shell, in which the 

 teeth or hinge is placed, the summit or 

 apex ; we regard it as the most elevated 

 part of the beaks. Beak, the pointed ter- 

 mination, apex, or tip of the protube- 

 rances last-mentioned, and which, in ma- 

 ny shells, turn spirally downwards, or ob- 

 liquely, so that the beak itself is seldom 

 the most elevated part of the shell ; though 

 it is so sometimes, as for instance in the 

 mytilus edulis, or common muscle. Sides, 

 the lateral parts of the valves distinguish- 

 ed by the epithet of right and left side ; 

 in common language, the two valves of a 

 shell are called the sides, but it is not un- 

 derstood as a term in conchology in this 

 view. Margin, or limb, the whole cir- 

 cumference or outline of the shell, when 

 laid flat down on one valve. Disk, the 

 convex centre of each valve, or exterior 

 surface. Anterior slope, that part of the 

 shell in which the ligament is situated; 



in the front view of the anterior slope, the 

 beaks fall back, or behind. Posterior 

 slope, that immediately opposed to the 

 former, and in which the beaks of the 

 shell turn forward. Lunule, the lunulat- 

 ed depression below the beaks, either on 

 the anterior or posterior slope, and some- 

 times on both ; they may be distinguish- 

 ed under the appellation of anterior or 

 posterior lunules, according to the slope 

 in which they are situated. Cartilage of 

 the hinge, called also the ligament of the 

 hinge, the substance of a flexible, fibrous, 

 and somewhat horny nature, by means of 

 which the two valves are united near the 

 beak, and by which also the shell is open- 

 ed at the will of the inhabitant. Ears, the 

 lateral processes near the beaks, as in 

 the scallop tribe : those occur either on 

 one side, or on both. Ligament perfora- 

 tion, the opening, or aperture, through 

 which the ligature of the animal passes, 

 as in the anomia genus, by the assistance 

 of which it fastens itself to the rocks, 

 or other bodies ; in some it is situated 

 in the flat valve, in others at the beak 

 of the gibbous valve. Length and breadth 

 of the shell. The length is measured 

 from the cartilage or beak to the margin 

 below, the breadth is of course taken in 

 the opposite direction. The breadth of 

 many bivalve shells exceeds their length : 

 some remarkable instances of which oc- 

 cur in the solen tribe. Hinge, the 

 point of union between the two valves, 

 formed by the connection or articula- 

 tion of the teeth in both valves, or by the 

 teeth in one valve, fitting into hollow 

 sockets in the valve opposite. The a- 

 mazing variety of structure, observable 

 in the hinge of different tribes of shells, 

 renders this one of the most essential 

 characters in the general definition of 

 shells. The teeth in some are small and 

 numerous, in others thick, solid, and few 

 in number, or sometimes single, long, 

 spatuliform, laminiform, acicular, &c. the 

 principal of which may be divided into 

 inarticulate hinge, when only furnished 

 with callosities, or having no visible teeth ; 

 articulate, when it has teeth, but only a 

 small number ; and multarticulate, when 

 the teeth are numerous. Cicatrix, the 

 impression on the inside of the valves, 

 indicating the point of connexion between 

 the muscles of the animal and its shell. 

 The muscles acting on the cicatrix close 

 the shell. In some kinds, as the common 

 oyster for example, there is only one such 

 muscular impression in each valve ; in 

 others there are two, and some have 

 more. The cicatrix is not of the same 



