BIVALVE. A shell composed of two equal, or nearly equal principal parts, each part having a separate nucleus, turning upon each other by means of a hinge. The class Conchifera of Lamarck, Acephalophora of De Blainville severally include the whole of the bivalve shells; the latter name being derived from the fact that the animals have not distinct heads, and neither eyes nor tentacula. All bivalve shells are marine or fresh-water. They form the class Dithyra of Aristotle. It may be observed that some of the Acephalophora, the Pholades, for example, have small testaceous pieces fixed on the hinge, which are called accessary valves. These are still fairly bivalve shells, although the genus Pholas has been placed by some writers among the multivalves. BOAR'S TUSK. A common name given to shells of the genus Dentalium. One particular species has received a specific name in accordance with a supposed resemblance, namely, Dentalium Aprinum, (of a Boar.) BONELLIA. Desh. A genus formed, in the first instance, for the reception of Bulinus terebellatus, Lam. which Mr. G. B. Sowerby, in his Genera of Shells, united with the genus PYRAMIDELLA. M. Deshayes, however, in his new edition of Lamarck, makes the genus Bonellia include several species which I have arranged in the genus Eulima. From the remarks of M. Deshayes, tom. 8, p. 286, 287, we are led to suppose that the estimated difference between Eulima and Bonellia consists in the latter having the axis perforated; or in other words, umbilicated. After remarking "que Mr. Sowerby, junr. confond deux choses bien distinctes, sous le nomme d'Eulima," M. Deshayes gives the following description of his genus, (translated) "shell turriculated, smooth, polished, with the apex acute and laterally inclined; axis perforated throughout its length; aperture small, entire, angular at the extremities; columella simple and without folds; outer lip thin, simple, nearly parallel with the longitudinal axis." That author further remarks, "Mr. Sowerby, junr. à signalé cinque espèces vivant, que nous rapportons à notre genre." (Sowerby, junr. Conchological Illustrations, parts 52 and 53; 50, Great Russell Street, Bloomsbury.) The species thus selected are E. splendidula, E. marmorata, E. interrupta, E. imbricata, E. brunnea; the two last of which have the umbilicus so inconsiderable, as to be scarcely distinguishable from other species, which M. Deshayes has left in the genus Eulima, and which have a slight hollow, almost approaching to a perforation, behind the columella. Eulima marmorata, (Bonellia, Desh.) is figured in the plates, fig. 348. BODY WHORL. The last whorl, constituting the bulk of the shell. BORELIS. Montf. MELONIA, Bl. A genus of microscopic Foraminifera.