boar's tusk. 83 



BILOBATE. Having two prominent parts, as the outer lip of 

 Rostellaria Pes-Peleeani, fig. 404. 



BIPARTITE. Composed of or divided into two parts ; double ; as 

 the valves of Platylepas, fig. 19, each of which has a septiform 

 division in the centre ; also the area on the hinge of Spondylus. 

 See Frontispiece. 



BIROSTRA. Sw. A genus composed of species of Ovulum, 

 which have elongated extremities, as, for instance, Ovulum Volva, 

 fig. 442. 



BIROSTRITES. Lam. (Double Beak.) A fossil formerly con- 

 sidered as a distinct bivalve shell, with conical umbones, and 

 placed in the family of Rudistes by Lamarck, but now known to 

 be an internal cast of Sphserulites, fig. 196. 



BISIPHYTES, Described by De Montfort as resembling a Nautilus, 

 but having two distinct siphons. As no such fossil species is now 

 known to Naturalists, it appears probable that De Montfort 

 having a specimen of some Nautilus, with an accidental depression, 

 took it for a second siphon. 



BITHINIA. Gray. A genus described as differing from Paludina, 

 in having the operculum shelly, and the mouth of the shell 

 thickened internally. Paludina impura, Auct. Fig. 537. 



BITOMUS. Montf. A microscopic shell, deriving this general 

 appellation, from the appearance of a double aperture. 



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 multi valves. 



BOAR'S TUSK. A common name given to shells of the genus 



G 2 



