CONCHIFERA. 
433 
hollow under the beak, always perceptible, and not forming a 
tendinous chord beneath the shell. 
TRIBE I.-OSTRACEA. 
Ligament placed either interiorly or nearly so ; shell irregu¬ 
lar in form, foliaceous, and sometimes papyraceous. 
* Papyraceous, thin shells, with an interior ligament. 
Genus 10.—ANOMIA.— Linnaeus. 
Generic Character. —Shell inequivalve, irregular, operculated, 
adhering by the operculum ; under valve flattened, with a large 
hole near the umbo, through which protrudes a testaceous, 
straight, elliptical operculum, or plug, by which the shell ad¬ 
heres to extraneous bodies ; upper valve larger, concave, and 
entire. 
Anomia undulata. — The Waved Anomia. 
Plate LXXXVII. fig. 2. 
Suborbicular, pellucid, with strong, irregular, undulated longi¬ 
tudinal stria?, and some transverse concentric ones ; outside 
yellowish dirty green ; inside fine green ; the ligament perfora¬ 
tion large, ovate; muscular impression star-shaped. Two inches 
broad. Inhabits the British coasts. 
Genus II.—PLACUNA.— Lamarck. 
Generic Character .—Shell free, subequivalve, irregular, very 
flat; hinge interior, with two short longitudinal convergent ribs, 
and divaricated beneath, which fit into two grooves in the op¬ 
posite valve, held together by the ligament. 
Piacuna placenta. — The Placenta Placuna. 
Plate LXXXVII. fig. 3. 
Suborbicular, flat, white, and pellucid, with fine longitudinal 
striae, slightly decussated. Four inches long. Inhabits the 
coasts of China. 
** Shell foliaceous, 
terior ligament. 
frequently ponderous, with a semi-in- 
2 N 
