The Cambridge Natural History Volume 3 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1895 Excerpt: ...has a paucispiral operculum with hollow whorls, deceptively like a Planorbis; it fits over the aperture instead of into it. In Livona and most Trochidae the operculum is cartilaginous and multispiral. In Strombus it is narrow, curved, and often serrated like a leaf on one of the edges; in Conus it is narrowly oblong and rather featureless; in Littorina, paucispiral and always cartilaginous, i J. E. Gray, Phil. Trans. 1833, p. 812. In many cases (e.g. Paludina) there is no true spiral form, but the striae are concentric to a nearly central nucleus, and thus give the appearance of a spiral. The evolution of the operculum in Navicella from Nerita has already been illustrated (p. 10). Neritopsis has a very remarkable operculum, the striated Pyrula Purpura Littorina Anlopoma Torinia Neritopsis Conus x J x 3 Strombus x 2 Fro. 183.--Various forms of opercula. appendage of which locks behind the columella of the shell, like the tooth in the opercula of the Neritidae. Terms employed to denote various parts of the Bivalve Shell.--The umbo, or beak, is the apex of the hollow cone, of which each valve may be regarded as consisting. This apex is usually more or less twisted: it is markedly spiral in Isocardia, Diceras, some Chama, and especially Requienia, while in Pecten, Lepton, and others the spiral is altogether absent. As a rule the umbones point forward, i.e. towards the anterior end of the shell. In Donax, Nucula, and Trigonia, however, they point backward. The umbones are generally more or less approximated, but in Area they are widely separated. An equilateral shell is one in which the umbones are more or less central with regard to its anterior and posterior portion, while in an inequilateral shell the umbones are much nearer one end than the other. On the oth...