A Manual of the Mollusca, Or, a Rudimentary Treatise of Recent and Fossil Shells Volume 3 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1851 edition. Excerpt: ...the byssal muscles are equal to the great adductors in size. In a few rare instances the muscles are fixed to prominent apopiiyces. The falmfonn processes of Plwlas and Teredo (Pl. XXIII. fig. I9, 26) are developed for the attachment of the foot-muscle; the posterior muscular ridge of Diceras and Cardilia resembles a lateral tooth, and in the extinct genus Radiolites both adductors were attached to large tooth-like processes of the opercular valve; but, as s rule, the muscles deposit less shell than the mantle, and their impressions deepen with age. The pallial line (fig. 177, m) is produced by the muscular fibres of the mantle-margin; it is broken up into irregular spots in the monomyary bivalves, and in Sazieava and Panoprea Norveyica. The siplzonal impression, or pallial sinus (fig. 14, p. 26, ) only exists in those shells which have retractile siphqns; its depth is an index to their length. The large combined siphons of Mya (fig. 170) are much longer than the shell; and those of some Tellinidw three or four times its length, yet they are completely retractile-The small siphons of Cyelua and Dreissena cause no inflection of the pallial line. The form of the sinus is characteristic of genera and species. In the umbonal area (within the pallial line) there are sometimes furrows produced by the viscera, which may be distinguished from the muscnlar markings by absence of polish and outline. (See Lucina, Pl. XIX. fiv. 6.) D Fossil bivalves are of constant occurrence in all sedimentary rocks; they are somewhat rare in the oldest formations, but increase steadily in number and variety through the secondary and tertiary strata, and attain a maximum of development in existing seas. Some families, like the Q1/prinida: and Lueim'dcents are mor