124 



CONCHOLOGY. 



as in the Venus, zig-zag ; internal, when it is positively 

 altogether concealed, as in the Mactra, Crassalclln, 

 and to a certain extent in the oyster. As to its being 

 flat, round, short, long, &c., it speaks for itself. 



The last mode of connexion between the valves of 

 shells is called the hinge (cardo), which may be de- 

 fined as a particular arrangement of differently formed 

 prominences, or cavities on each valve corresponding 

 reciprocally with each other. Authors call that the 

 thickest portion of the circumference of the valve in 

 which most frequently these cavities or teeth are 

 situated ; this is the cardinal margin (margo cardinalis) ; 

 the cardinal blade or ridge (disscpimentum cardinalis) is 

 that portion of the edge which exhibits the teeth. 

 Examining shells with regard to this arrangement, 

 they are called toothless (acardis) when no trace 

 exists of cavities, projections, or ligament, but of these, 

 perhaps, only one example can be offered, it is the 

 Lingula, When there is only one protuberance on 

 the part termed the hinge, more or less elongated 

 and irregular, it is called callus (callosa). When the 

 cardinal ledge, instead of being vertically placed, as is 

 in most instances the case, is horizontally enlarged 

 by a projection fitting into a corresponding cavity, 

 placed beneath the summit of the other valve, or 

 under a similar projection, Linnaeus called it dens 

 vacuus or depressns ; but erroneously, not being a true 

 tooth, and only serving for the insertion of the liga- 

 ment. In all other shells provided with a true hinge, 

 it is necessary to observe if it is exactly similar on 

 each valve, in the first instance, it is termed therefore 

 similar (similis), and in the second dissimilar. The 

 position of the hinge, generally considered, also re- 

 quires some particular denominations, which are nearly 

 the same as applied to the summits ; oval, dorsal, anal, 

 &c., as they may be situated on the back, posterior, 

 anterior, &c. In the different portions of the hinge 

 it is formed complete by means of eminences and 

 cavities, the one called teeth, the other indentations 

 (fossula) ; when the hinge is formed by one tooth or 

 protuberant portion, as in the Mya, it is called the 

 Sinus by Linnaeus. These eminences or correspond- 

 ing cavities with relation to the summit may generally 

 be denominated in the same manner as the entire 

 hinge. The cardinal teeth (denies cardinales) are those 

 placed immediately beneath the summits, and are 

 usually the principal ones. The lateral teeth (denies 

 laterales) are, on the contrary, those of less importance, 

 and more or less separated either before or behind 

 the summit ; those placed nearest to the lamella are 

 called dens anticus; and the dens posticiis are those 

 on the side of the ligament or shield, thus reversing 

 the Linnaean position. The direction in which these 

 teeth are placed distinguishes them into vertical, ob- 

 lique, longitudinal, divergent, or convergent. In the 

 Lyrena they are nearly vertical, oblique in the Venus, 

 longitudinal in the Cardita, divergent or convergent, 

 according to the point of view taken from the summit, 

 as in the Mactra. The mode of junction of these 

 teeth establishes the terms inserted (intrans}, which 

 is where one penetrates between two others. Alter- 

 nate (alternus), that which crosses another obliquely, 

 as in the Cardium; articulated (inscrtus), when the 

 hinge formed by them is produced from a reciprocal 

 arrangement inverse on each valve, as exists in the 

 greater number of bivalves. The particular form of 

 the teeth is expressed by lamellar (lamellosa) or lon- 

 gitudinal when they are very long ; and much com- 

 pressed if short or thick ; when on the contrary, straight 



or curved, simple or bifid, smooth or striated, accord- 

 ing to circumstances, too obvious to need further 

 explanation. The number of teeth is sometimes 

 important to observe, though they are not always 

 uniform in the same species of molluscs. In multi- 

 valve shells, according to the modern system of clas- 

 sification, such as were included by Linnaeus from 

 being more or less complete tubes, which might 

 accompany or even entirely envelope the two valves 

 of a bivalve mollusc, were by him sometimes 

 called accessory pieces, have been excluded, and only 

 those completely visible are so called. Multivalve 

 shells are constantly produced by animals which may 

 be called intermediate between the mollusca and the 

 entomozoria, while those of the Pholas, Teredo, &c., 

 are true malacozoaria. 



They are also very few in number, and it is scarcely 

 necessary to dwell particularly on the denominations 

 of their parts, as the greater portion of them are the 

 same as those of bivalve shells ; they may be called 

 with regard to the position of their parts or valves, 

 articulated, lateral, ante or post lateral, dorsal or ven- 

 tral, but one group established by Lamarck, under the 

 name of coronal or sub-coronal molluscs, requires a 

 more particular notice. In these the portions consti- 

 tuting the shell being disposed regularly round a com- 

 mon axis, are firmly knit together by their margins, 

 so as to form a complete cavity or space in the centre, 

 closed or open inferiorly, and shut superiorly, by a 

 small number of pieces of various forms, the total of 

 which is called their operculum, or lid ; the form and 

 number of principal pieces, as well as those thus 

 termed the operculum, varies so much that they merit 

 distinct terms to designate them. The support (basis) 

 is always monotomous, and may be simply membran- 

 ous, irregular, or patelliform, in which case it may 

 easily be confounded with the Limpet. The terminal 

 or operculated portion is at least ditomous, because 

 it is always in pairs and symmetrical, but then each 

 lateral piece is most frequently divided into two valves, 

 the one dorsal, the other ventral, according as they 

 correspond to the analogous coronal pieces. This 

 operculum is called articulated, when it evidently 

 touches the coronal portion ; it is inarticulated when 

 entirely surrounded by the membranous portion of 

 the aperture. The principal portions of these shells 

 have occasioned their being called coronal, because 

 they form a sort of crown round the body of the ani- 

 mal. The number of pieces forming what is termed 

 the operculum usually amount to six, but they vary 

 much sometimes, being only four in the usual state of 

 these valves ; each is divided on its surface into two 

 triangular areas, the one in relief, the other hollow. 

 And in the interior of these it is found that they are 

 doubled in their superior or anal half by a vertical 

 plate or separation, the extent and form of which is 

 very important in distinguishing this singular species 

 of shell. A reference to the article Balanus will also 

 point out some other peculiarities not yet satisfactorily 

 explained. Upon this subject, as well as many others 

 connected intimately with the study of the science of 

 conchology, we have been constantly tempted to en- 

 large ; but a want of space must preclude us from 

 dwelling longer, as a volume might be written, filled 

 with ingenious hypotheses, and well-grounded deduc- 

 tions ; our object, as has been more than once stated, 

 being to induce a deeper research in others, raihor, 

 than to broach our own opinions, or to write a volumin- 

 ous treatise on the subject. So numerous are the pub- 



