Regularity.
A great number of Bivalves are extremely regular in their form. These are generally locomotive, and consequently free from those obstructions in growth occurring to stationary shells, which being confined in a particular position, or to a particular spot, modify their shape according to the substance with which they come in contact, and thus become irregular. This is generally the case with shells which are attached to submarine substances, such as Spondyli, Oysters, &c.; and the degree of irregularity will depend upon the extent of surface involved in the attachment. In the case of fixed shells, the attached valve is usually termed the under valve, and the other which moves freely upon the hinge, is termed the upper valve.
Form and Proportions.
Bivalves are said to be equivalve when the two valves correspond in extent, breadth, and thickness; and of course inequivalve in the contrary case. They are equilateral when a line drawn from the umbones to the ventral margin would divide the shell into two nearly equal parts; and of course inequilateral in the opposite case, which occurs in the great majority of instances.
A Bivalve is said to be compressed, when the distance is small from the most prominent part of one valve to that of the other. It is cylindrical when lengthened, and more or less rounded in its breadth, as in Lithodomus (fig. 161 in the plates). It is cordiform when the shape presents a resemblance to an imaginary heart, as in Cardium cardissa (fig. 122 in the plates), and in the Isocardia (fig. 126 in the plates). It is linguiform when it resembles a tongue in shape, as in Vulsella (fig. 185 in the plates); rostrated when it protrudes at either extremity, and terminates in a kind of point, as in Sanguinolaria Diphos (fig. 99 in the plates); truncated when it ends in a square or angle, as if cut off; an example of which may be seen in Solen (fig. 60 in the plates).
Other Bivalves are distinguished as being auriculated, having processes flattened and expanded on either side of the umbones, as in Pecten (cut, fig. 82). When there is one of these on each side of the umbones, it is bi-auriculated; when only on one side, it is uni-auriculated. When the expansion is very broad, as in Unio alatus (fig. 142 in the plates), and in the Hammer Oyster (cut, fig. 83), the term alated is used.
Fig. 82, auriculated; 83, auriculated, alated.
With regard to these alated species of Uniones, it is necessary to observe that they are also "adnate," as it is termed; the two valves being joined to each other by the dorsal edge of the expanded parts, and united so completely in substance with each other, that they cannot be separated without being broken. Many other terms are used to express difference in Bivalves, but being generally applicable to Univalves and Multivalves, as well as to them, they will be found explained at large in the alphabetical part of the work.
MULTIVALVE SHELLS.