116 DIMYARIA. GASTROCHENA. 
greater or lesser number of accessory valves, situate near the 
fulcrum of the hinge, and connected with the shell only by 
the epidermis, which passes over them ; each valve furnished 
with a long curved, flat, tooth-like testaceous process, pro- 
jecting from the interior of the shell, immediately within the 
umbones; in some species this is expanded, and spoon- 
shaped ; anterior dorsal margin near the beaks reflected, 
close, and flattened down upon the umbones in some species, 
and in others a second margin is produced, situate remote 
from the first, with the intervening space divided by a series 
of transverse septa; two principal impressions, formed by 
the adductor muscle, one of which is placed on the reflected 
margin over the beaks, and the other intermediate, between 
the umbones and the posterior side; muscular impression 
with a large sinus in its narrower part, the impression being 
somewhat expanded near to the sinus. 
Pholas aperta. Plate X. figs. 20, 21, and 28. 
The Pholades differ from the Teredines, chiefly in the latter having a testa- 
ceous tube situate behind its valves, in having no accessory appendages or 
valves, and when the valves are closed, they assume nearly a globular form. 
Turton’s genus, Xylophaga, is provided with accessory valves, but is destitute 
of the testaceous tube, and the internal shelly tooth-like process of Teredo and 
Pholas. 
The Pholades inhabit the ocean, and are found in the seas of all countries, 
burrowing in limestone rocks, wood, or clay. They are rare in a fossil state, 
and those known have been found in the English Crag, in the Calcaire-grossier 
of France, and its contemporaneous formations in Touraine and Italy. Some 
of these have been found in the cavities which they themselves have made. 
TRIBE IV.—TUBICOLA. 
Animal contained in a testaceous sheath, distinct from its 
valves, incrusted entirely or in part in the wall of this tube, 
or projecting outwards. 
Genus LVI. — GASTROCH ANA. — Spengler. 
Generic Character. — Shell equivalve, inequilateral, some- 
what wedge-shaped ; anterior side rounded, when viewed in 
front, and posteriorly acuminated ; anterior side gaping 
widely, its aperture being subovate, and acute behind ; 
hinge marginal and linear, destitute of teeth, but in their 
stead a small laminated appendage, emanating from the 
umbo, allied to the same tooth-like process in the genus 
Pholas ; ligament external. 
Gastrochena contorta. Plate XI. fig. 14. 
This shell is enclosed in a testaceous, irregular, claviform 
