6 
Fistulana. — Sheath tubular, mostly testaceous; the posterior closed and turgid; the 
other end attenuated and open at the summit, inclosing a free bivalve shell; 
the valves of which are equal, and gape when closed. 
Plate II. Fig. 8. F. gregata. 
(a) One of the bivalves. 
(b) One of the side valves. 
Septaria. — T ube testaceous and very long, gradually diminishing towards the anterior 
end; the interior divided by arched partitions, usually incomplete: the anterior 
extremity terminated by two other slender tubes, which are not divided inter- 
nally. 
Plate II. Fig. 9. S. arenaria. (Serpula polythalamia. — Linn.) 
Teredina. — Sheath testaceous, tubular, cylindrical; the posterior extremity closed, 
shewing the two valves of the shell ; the anterior end open. 
Plate XXI. Fig. 1. T. personata. Fossil. 
Teredo. — Tube testaceous, cylindrical, flexuous, open at both ends, not belonging to 
the shell, and covering the animal. Shell bivalve, situated posteriorly on the out- 
side of the tube. 
Plate II. Fig. 10. T. navalis. The testaceous tube. (Idem.— Linn.) 
(a) One of the bivalves. 
( b ) One of the terminating side valves. 
(c) A section of the anterior part of the tube. 
PHOLADARIA 2 Genera. 
Shell without a tubular sheath, having accessory pieces which do not belong, to the valves, and gapes 
anteriorly. Ligament external. 
Pholas. — Shell bivalve, equivalve, transverse, gaping at both sides, having various acces- 
sory testaceous pieces, affixed above or below the hinge. The inferior or poste- 
rior margin of the valves reflected outwards. 
Plate II. Fig. 11. P. Candida. (Idem. — Linn.) 
Gastrocilena. — Shell bivalve, equivalve, rather wedge-shaped, gaping very much; the 
anterior aperture large, oval, oblique ; scarcely any aperture posteriorly. Hinge 
linear, marginal, without teeth. 
Plate II. Fig. 12. («) G. modiolina. Front view. 
( b ) Back view. 
SOLENACEA. __ 3 Genera. 
Shell without accessory pieces, and gaping only at the lateral extremities. L igament external. 
Solen. — Shell bivalve, equivalve, transverselyeiongated; gaping at both sides; beaks 
very small, not projecting. Cardinal teeth small, the number variable, sometimes 
none ; rarely diverging ; more rarely inserted in pits. Ligament external. 
