FISTULANA. DIMY ARIA. 117 
tube, situate at its broader extremity; it is open and 
attenuated anteriorly, with an oblong, bilobate aperture, 
which is nearly subdivided by a projecting septum, that 
does not quite reach across the opening ; these serve for the 
passage of the two tubes of the animal ; the posterior end 
of the tube is closed. This club-shaped tube is found either 
within the perforated cavities of rocks, or in old shells or 
corals, the testaceous tube always protruding beyond the 
surface, 
The Gastrochena are readily distinguished from the Pholades by their 
testaceous tube, and by wanting accessory valves; they seem the connecting 
link. between the Pholadaria and Tubicola. They differ from the Fistulane 
in the oval form of the valves, and in the curvature of the tube, from Clava- 
gella and Aspergellum in both valves being free, and from Galcomma in the 
shell being free and oblique. 
The shells of this genus inhabit the ocean, and only one fossil species has 
been found, attached to the inside of other fossil shells in the Calcaire-grossier 
at Grignon, France. 
Genus LVII.— TEREDINA. — Lamarck. 
Generic Character.— Shell orbicular, and entirely ex- 
ternal, equivalve, inequilateral ; umbones greatly incurved, 
and covered by a somewhat quadrangular, accessory pro- 
cess, which seems to be fixed to the valves in front of the 
beak, with a subulate process in front, and gaping at both 
extremities ; anterior opening angular at the back, and the 
posterior rounded in front ; tube thick, fistulous, posterior 
extremity smaller, and open, and nearly divided into two 
from an interior projection on both sides, and provided with 
an operculum ; anterior termination of the tube entirely 
closed by a trapezoidal plate, which fills up the space left 
by the sinus in the two valves. 
The posterior portion of the tube is of a different con- 
sistence from the anterior part, having a horny texture and 
appearance ; the interior of the valves is thickly lined with 
the same testaceous matter as the tubes. This testaceous 
substance is generally so much thickened in front, that it 
almost entirely conceals the tooth-like processes. 
Teredina personata. Plate X. figs. 23 and 25. 
The shells of this genus are known only in a fossil state, and seem to have 
been gregarious. They are found plentifully in the Ferruginous sand, 
Genus LVIII, — FISTULANA, — Bruguiere, 
Generic Character. — Shell equivalve, inequilateral, 
transversely elongated, and gaping widely at the basal 
