6 
Fistutana.—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.—Tube 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.) 
TrEREDINA.—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. 
(6) One of the terminating side valves. 
(ce) 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. 
Puotas.—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.) 
GAsTRocHzNA.—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. (4) G. modiolina. Front view. 
(6) Back view. 
SOLENACEA.—3 Genera. 
Shell without accessory pieces, and gaping only at the lateral extremities. Ligament external. 
SoLeN.—Shell bivalve, equivalve, transversely elongated; 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. 
