. Elements of conchology / Prepared for the use of schools and colleges. Mollusks. 08 PARTS OF BIVALVE SHELLS. When upon a similar division, a slight difference is observed in the two valves, the shell is termed subequilateral; but where the difference is well marked, it is an inequilateral bivalve (Jig. 117). When the apex is bent, as is commonly the case, out of the transverse plane, it is always directed more or less towards the anterior part of the shell; if such a bivalve shell as the Cytherea, or Isocardia, be held before the observer, with the umbones directed forward, and the hinge abo
. Elements of conchology / Prepared for the use of schools and colleges. Mollusks. 08 PARTS OF BIVALVE SHELLS. When upon a similar division, a slight difference is observed in the two valves, the shell is termed subequilateral; but where the difference is well marked, it is an inequilateral bivalve (Jig. 117). When the apex is bent, as is commonly the case, out of the transverse plane, it is always directed more or less towards the anterior part of the shell; if such a bivalve shell as the Cytherea, or Isocardia, be held before the observer, with the umbones directed forward, and the hinge above,—in the position, in fact, in which the living animal would place itself if it were creeping forwards from the observer,—the right valve will of course correspond with the right hand of the observer, and the left with the left. Superior or dorsal border. s.^ ^, A^. A J. i"P Fig. 117.—LEFT VALVE OF A CYTHEREA. The annexed figure (117) represents the left valve of &. Cytherea,—the common clam belongs to this group, and would answer our purpose as well. You see that the superior or dorsal margin is on the side where the hinge is placed, and the ventral or inferior border opposite to it; A, is the anterior margin or slope; P, the posterior margin or slope; the umbo is marked " Beak," and s, points to the summit or apex of the beak ; /, shows the situation of the Ivnule. Now, if a bivalve in which the apices have a spiral twist, as in the Isocardia {jig. 104), be placed in the above position, and compared with the univalve shell of the Purpura, it will be seen that the left valve corresponds with the ordinary or dextral spiral univalve, and the right valve to the perverse or sinistral univalve. When the circumference or margin of one valve fits exactly, at every part, that of its fellow, it is said to be " regular" or " entire;" but if it be notched at any part, so as not to come. Please note that these images are extracted from scan
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Keywords: ., bookcentury1800, bookdecade1840, booksubjectmollusks, bookyear184
