BIVALVIA. 299 
Generic Character. Shell convex, tumid, inequilateral, equivalved, presenting, with 
the valves united, an orbicular or nearly spherical outline, having a large angular 
opening in front, and a rounded or ovate aperture posteriorly ; covered or ornamented 
externally with ridges or striae, caused by the slightly reflected or thickened edges of 
the margin; a testaceous process or tooth, of a somewhat spatulate form, curves and 
projects inwardly from beneath the umbo; one distinct muscular impression ; ligament 
obsolete. 
Animal worm-shaped; mantle open in front; siphons very long, bifurcating at 
their extremities; orifices fringed; foot rudimentary. 
The valves are situated at the anterior extremity of an irregular, subcylindrical, 
elongated, and somewhat flexuous testaceous tube, which appears to be formed for the 
purpose of lining the cavity made by the animal. This tube, in some species, is large, 
thick, and heavy, while in others it is thin and semi-transparent; the variation, per- 
haps, depending upon a necessary protection against lateral pressure. At the posterior 
termination are two pennate or palmate opercula, called pallettes by Adanson, or 
calamules by Deshayes, by which the aperture is closed at the will of the animal, one 
probably employed as a lid to each siphon; and this end of the tube, in some species, 
is partitioned with segments or semi-camerations, but for what especial purpose has 
not been satisfactorily ascertained. The growth of the animal, and necessarily the 
enlargement of the valves, causes a corresponding increase in the size of the tube, 
which is lengthened at the anterior extremity, while the posterior part is kept on a 
level with, or a little beyond, the surface of the wood, so as always to keep the siphons 
free.* The addition of calcareous matter is made to the larger end only, which is 
kept open until the animal has attained to full maturity, when it is closed with a convex 
or dome-shaped termination. 
The habits of most of the species lead them to excavate timber of all kinds. 7, 
corniformis burrows in the husks of the cocoa-nut ; and 7° arenaria locates itself in mud. 
The tube of the latter animal (figured and described by Mr. Griffiths, in the ‘ Phil. 
Trans.’ for 1806, p. 269, from a specimen found on the N. W. Coast of Sumatra) mea- 
sured 5 feet 4 inches in length, and 9 inches at its greatest circumference ; the posterior 
termination was longitudinally divided into two separate tubes for about 8 or 9 inches, 
and the larger end was closed, where the animal had constructed two separate septa 
a few inches within. The character of a longitudinal division for the two siphons is 
sometimes observable in the European species of this genus, and they terminate di- 
vergingly ; but this, probably, is not constant, and only dependent upon circumstances. 
The tube of Z. arenaria presents, when fractured, a crystalline or radiating structure. 
The tubes of our fossil, and other European Zeredines, are composed of numerous 
* In the kindred genus Xylophaga, the animal does not furnish itself with a calcareous tube, as it 
never retreats far from the surface of the wood into which it has penetrated, consequently has no elongated 
siphons. 
