21 
the external channel. Ligament internal; the remains of former ligaments are 
seen outside in the channel. 
Plate XII. Fig. 6. S. yederopus. (Idem.—Linn.) 
Povopsts.—Shell inequivalve, subregular, adhering by the inferior beak, not auriculated, 
the lower valve largest, most convex, and the beak most produced: hinge without 
teeth. Ligament internal. 
Plate XXI. Fig. 5. P. truncata. Fossil. [Icon.—Encye. Method. Pl. 188. Fig. 6.] 
OSTRACEA.—5 Genera. 
Ligament internal, or demi-internal. Shell irregular, foliated, sometimes very thin. 
Turis Family is separated into Two Divisions. 
(a) Ligament demi-internal. Shell foliated, and often very thick. 
GrypuxA.—Shell free, inequivalve, the lower valve large, concave, terminated by a pro- 
jecting involute beak, the upper valve small, flat, and opercular: hinge without 
teeth; an oblong, arched cardinal pit; only one muscular impression in each 
valve. 
Plate XII. Fig. 7. G. angulata. 
Ostrea.—Shell adhering, inequivalve, irregular; beaks distant, becoming very unequal 
by age, and the upper valve gradually removed during the life of the animal: hinge 
without teeth. Ligament demi-internal, inserted in the cardinal pit of the valves; 
the pit of the lower valve increasing by age, sometimes to a great length. 
(1) Margin of the valves simple or wavy, not plicate. 
Plate XII. Fig. 8. O. edulis. (Idem.—Linn.) 
(2) Margin of the shells distinctly platted. 
Plate XII. Fig. 9. O. folium. (Idem,—Linn.) 
Vutsetia.—Shell longitudinal, subequivalve, irregular, free; the beaks equal: hinge 
with a prominent callosity in each valve, depressed above, shewing an impression 
of a conical and obliquely arched pit for the ligament. 
Plate XII. Fig. 10. V. lingulata. (Mya vulsella.—Linn.) 
(b) Ligament internal. Shell thin, papyraceous. 
Piacuna.—Shell free, irregular, flat, subequivalve: hinge internal, having two sharp 
longitudinal ribs in one valve, approximate at their base, and diverging in form of 
a V; and, in the other valve, two impressions which correspond with the cardinal 
ribs; to which is attached the ligament. ; 
Plate XII. Fig. 13. P. placenta. (Anomia placenta.—Linn.) 
Anomia.—Shell inequivalve, irregular, operculated, adhering by the operculum: lower 
valve perforated, generally flattened, having a hole or channel at the beak; the 
other valve rather larger, concave, entire. Operculum small, elliptical, osseous, 
fixed to marine substances. 
Plate XIII. Fig. 1. A. ephippium. (Idem.—Linn.) 
(a) Operculuin of ditto. 
