21 
the external channel. Ligament internal ; the remains of former ligaments are 
seen outside in the channel. 
Plate XII. Fig. 6. S. gmderopus. (Idem. — Linn.) 
Podopsis. — 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. — Encyc. Method. PI 188. Fig. 6.] 
OSTRACEA.__5 Genera. 
Ligament internal, or demi-internal. Shell irregular, foliated, sometimes very thin. 
This Family is separated into Two Divisions. 
(a) Ligament demi-internal. Shell foliated, and often very thick. 
Grypilea. — 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 plaited. 
Plate XII. Fig. 9. 0. folium. (Idem. — Linn.) 
Vulsella. — 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. 
Placuna. — 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) Operculum of ditto. 
