122 



CONCHOLOGY. 



^deration of the different terms applied to the spire, 

 and other parts of univalve shells, would lead into a 

 description of terms generally used in every element- 

 ary treatise on conchology ; but as it would occupy 

 a larger space than can be devoted to the subject 

 in a work of this kind, we will proceed to point out 

 some of the leading characteristics of bivalve shells, 

 as we have already done with the univalves. And 

 here, as with them, we will first of all point out the 

 established position from which the relative terms are 

 taken, which is here figured. 



Fig. 1. 



Fig. 2. 



Fig. 3. 



Fig. t, a, b, length of the shell; e, d, height; e, lunula, 

 above which is the summit; rf, the ventral or inferior edge. 



Fig. 2, the line across marks the thickness of bivalves. 



Fig. 3, a, anterior extremity ; b, posterior; c, d,e,f, muscular 

 impressions; g, lower edge of the left valve. 



Viewing, therefore, a bivalve shell, composed as it 

 were of one piece, we shall explain that which is un- 

 derstood by long, elongated, cylindrical, transverse, 

 thick, compressed, or very thin. We have already 

 assumed as a point from which we shall invariably 

 depart, that we suppose the shell covering the animal 

 and walking away from the observer (although in fact 

 many of these molluscous animals do not move from 

 the place in which they were born) ; some others, it is 

 true, assume positions in which the head even is down- 

 wards, when the shell would be placed on its edge, in 

 such a way that the summits would be nearly always 

 upwards, and very rarely forward, with the ligament 

 between the summit and the observer. In this posi- 

 tion the part opposed to the summits will be inferior, 

 and the two extremities of the diameter perpendicular 

 to this direction, will be one anterior and the other 

 posterior. Linnaeus, Brugniere, Lamarck, Bosc, and 

 many other naturalists, have described shells alto- 



gether imagined in an opposite position, that is to My, 

 resting on their summits, the aperture upwards, and the 

 ligament forward. According to this position, the 

 height of a shell will be taken from the vertical dia- 

 meter extending from the summits, or from the liga- 

 ment to the inferior or abdominal edge, supposed to 

 touch the place on which the shell rests ; that is the 

 length with Linnaeus and others. The thickness, 

 (crassitude), will be indicated by the transverse dia- 

 meter of the most inflated part of the two valves, 

 whence the right valve will really be that which cor- 

 responds with the right side of the animal, and the 

 same with the left. We should then name the back 

 of the shell, or the superior margin, that which in fact 

 corresponds with the back of the animal in which or- 

 dinarily the summit is placed, but still more frequently 

 the ligament. The opposite side will be the abdomi- 

 nal side, or inferior margin, that is its real base. In 

 this light Cuvier and others have viewed the position 

 of bivalves, but it is the reverse with Linnaeus, Lamarck, 

 and many other naturalists. 



The circumference of the shell, or the line which 

 unites the four points just mentioned, forms the 

 margins of the shell ; in this position it would be easy 

 to understand what is meant by a long bivalve, &c. 

 It is long (longa) when the horizontal diameter is 

 much greater than the vertical, as in the Pholas, Mya, 

 &c. ; it is a transverse shell of the Linnaean school. 

 Height(#a),in a contrary sense, as in the Vulsella. Oval 

 (ovalis), when one of the diameters is but a little longer 

 than the other, as in the Venus. Round (rotundata), 

 when the two diameters are nearly equal, as in the 

 Pectens. Thick (crassa), when the transverse diameter 

 is as great as the others, upon which depends the 

 depth of the valves, as in the Cardium. Compressed, 

 thin, very thin (compre&sa), when this diameter is more 

 or less small in proportion to the others, as in the 

 Tellina. Cylindrical (cylindrica), when the longi- 

 tudinal diameter being very great, the two others are 

 nearly equal, as in some species of the Solen. Navi- 

 cular (navicularis), when the diameter, antero-posterior, 

 being evidently much larger than the two others, the 

 latter are nearly equal, which gives the shell a fancied 

 resemblance to the hull of a ship, particularly when the 

 dorsal margin is straight, as in the Arks. Cordiform, 

 heart-shaped, (cordiformis), when seen from behind, in 

 front, or at the side, it offers some resemblance to 

 the received form of a heart, as in the Isocardia, &c. 

 Triangular (triquetra), when the shell is as if truncated 

 at its anterior extremity, but much more frequently at 

 its posterior, so that a horizontal section, made of the 

 whole shell, would form a triangle, this is illustrated 

 in the Trigoida, Donor, &c. Tongue-shaped (lingui- 

 formis), when it resembles the form of a tongue, as in 

 the Vulsella. Rostrated, beaked (rostrata), when the 

 posterior extremity is much narrower than the anterior, 

 as in several species of Tellens. Truncated (truncata), 

 when it is, positively, as if one of its extremities were 

 cut off, as in several species of Donor. Eared (aun- 

 culata), when the margins of the shell, towards the 

 summit, are more or less dilated into certain portions, 

 termed ears. Inauriculated, when only one of these 

 appendages occurs. These ears are equal, or unequal, 

 when of the same size, or differing from each other 

 in size ; they are spiny, when their inferior margin 

 appears toothed ; obliterated, when they are but 

 slightly angular, as in the Lima. 



The valves may be regular or irregular ; they are 

 regular when they present an uniform shape, totally 

 independent of all exterior bodies, as is the case with 



