. A conchological manual. Shells. INTRODUCTION. 37 Umbones. These are the prominent points of the dorsal edge, where the growth of the shell commenced, and are called beaks, by some English writers. In some instances they are close to each other; in others they are rendered distant from each other by the intervention of areas in the hinge, as in Spondyli, &c. In Pectunculus they are straight; in Venus curved towards the anterior margin; in Isocardia, spiral; in Chama, decumbent; in Diceras, free. In shells subject to external corrosion, the process commences at the Fig. 75, dista
. A conchological manual. Shells. INTRODUCTION. 37 Umbones. These are the prominent points of the dorsal edge, where the growth of the shell commenced, and are called beaks, by some English writers. In some instances they are close to each other; in others they are rendered distant from each other by the intervention of areas in the hinge, as in Spondyli, &c. In Pectunculus they are straight; in Venus curved towards the anterior margin; in Isocardia, spiral; in Chama, decumbent; in Diceras, free. In shells subject to external corrosion, the process commences at the Fig. 75, distant; 76, straight; 77, curved; 78, spiral; 79, decumbent; 80, free; 81, close. GENERAL CHARACTER OF BIVALVES. 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 Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Sowerby, G. B. (George Brettingham), 1812-1884; Henderson, J. B. (John Brooks), 1870-1923, former owner. DSI; Dall, William Healey, 1845-1927, former owner. DSI. London : Henry G. Bohn
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