When the _breadth_ is spoken of, the distance between the most convex parts of both valves, when closed, is intended; but when an expression implying _thickness_ is used, it refers to the substance of each valve: it is important to bear this in mind, as many persons have been misled by descriptions in which the distinction has not been attended to. Glycimeris (fig. 67 in the plates) is a _thick_ shell, but Anatina (fig. 69 in the plates) is a _broad_ one. _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. [Illustration: Fig. 82, _auriculated_; 83, _auriculated_, _alated_.]