710 



CONCHIFERA. 



The teeth are commonly simple and conical ; 

 occasionally they are flattened either lengthwise 

 or transversely. In a considerable number of 

 species they are grooved to different depths on 

 their summits, and the teeth are then said to 

 be bifid (e, Jig. 365). 



There are other parts still which present 

 themselves upon the cardinal edge, and of 

 which it is important to have a sufficient know- 

 ledge, namely, those destined for the implan- 

 tation of the ligament when it is external ; to 

 these parts the name of nympha is given. 

 These form two callosities more or less promi- 

 nent, which are seen along the posterior and 

 superior edge of the shell. When the ligament 

 is internal, it rests upon a cavity generally pro- 

 minent towards the interior of the valves, and 

 designated by the name of cuitteron or spoon- 

 shaped cavity. This cuilleron is generally 

 situated in the centre (c, d, fig. 367) of the 



Fig. 367. 



hinge ; sometimes, however, it becomes a little 

 oblique, elongated, narrower, and runs in the 

 direction of the posterior and superior edge. 



When we direct our attention to the external 

 forms of the bivalve shells, we observe numerous 

 modifications, of the principal of which it is 

 necessary to take some notice. In a consi- 

 derable number of species the two valves are 

 alike, when the shell is said to be equivalved. 

 When one of the valves is larger than the other 

 it is of course inequivulved ; to constitute it so 

 it is not necessary that the shell should be 

 irregular. A regular shell is that which at 

 liberty always presents the valves alike in all 

 the individuals of the species ; an irregular 

 shell is not only inequivalved, but farther, the 



whole of the individuals of the same species 

 are not exactly of the same form, and want the 

 same peculiarities of external conformation 

 generally. The Oysters are inequivalve and irre- 

 gular shells ; the Corbules are inequivalve and 

 regular shells ; the Venus and many others are 

 perfectly equivalved and regular ; the Placunes, 

 % to choose a particular example, are in like 

 manner equivalved but irregular. The length 

 of a shell is always calculated from the summit 

 of the hooks to the inferior edge. All that are 

 of greater length than breadth are entitled 

 longitudinal, ( Mytihts, Pinna, &c. Jig. 353) ; 

 and all that are of greater breadth than length 

 are named transversal : the breadth is estimated 

 by a line passing from the anterior to the 

 posterior extremity, and cutting the posterior 

 axis of the shell at a right angle, (Solen, Tel- 

 Una, &c. The number of transverse bivalve 

 shells is very great : Jig. 367). If the position 

 of the hooks with relation to the transverse 

 and longitudinal lines be considered, the shell 

 is said to be symmetrical, when, the hooks 

 being in opposition, the anterior segment is 

 equal to the posterior, and of the same form 

 in consequence of this symmetry ; a perfectly 

 symmetrical bivalve shell might in fact be 

 held to be composed of four similar parts ; but 

 this perfection of symmetry, which exists in 

 many Brachiopods, never appears among the 

 conchifera properly so called, even those 

 which are the most symmetrical in external 

 character, as certain Petuncula, Jig. 358, would 

 be more correctly designated as sub-symme- 

 trical. When the hooks are inclined to one 

 side of the shell, and divide it into two equal 

 parts, it is said to be equilateral. But if the 

 hook be carried further forwards than back- 

 wards, so that one of the sides of the shell 

 then becomes larger than the other, it is said to 

 be inequilateral. In the greater number of the 

 conchifera the two valves of which the shell 

 consists join each other accurately around their 

 whole circumference, in which case the shell 

 is said to be shut or closed. When, on the 

 contrary, the two valves present a vacancy 

 between them in some part of their circum- 

 ference, when they are approximated as nearly 

 as possible to one another, the shell is said 

 to be patulous. This open space is vari- 

 ously situated in different species, sometimes 

 in the anterior surface, rarely in the inferior 

 edge, but pretty frequently in the posterior 

 edge, especially in those species of the class 

 whose mantle is prolonged on this side into 

 one or two syphons. 



Surfaces of the valves. Every bivalve shell 

 has two surfaces, an external surface and an 

 internal surface. Various parts are distin- 

 guished on the external surface, the hooks, 

 the belly, the edges, the lunule, and the 

 corslet. 



External surface. 1 . The hooks. The 

 protuberant opposed parts, often inclined to- 

 wards the anterior side, and presenting an 

 apex of various degrees of sharpness or blunt- 

 ness, are thus denominated. W 7 hen these 

 hooks are very much inclined forwards, they 

 arc styled lateral. If they are particularly 



