MONOMYARIA. 125 



shaped, with two unequal tubercles within; under valve sub- 

 ventricose, with the scales radiating beyond the margin, its 

 cavity obliquely conical, and forming a ridge by a replication 

 of its margin ; inside partly striated ; hinge unknown. 



Section II Ligament not marginal, enclosed in a short 



hollow under the beak, always perceptible, and not forming 

 a tendinous chord beneath the shell. 



TRIBE I. OSTRACEA. 



Ligament placed either internally or nearly so ; shell ir- 

 regular in form, foliaceous, and sometimes papyraceous. 



* Papyraceous, tliin shells, with the Ugament internal. 



Genus 10. — ANOMIA. — Linnceus. 



Generic Character. — Shell inequivalve, irregular, opercu- 

 lated, adhering by the operculum ; under valve flattened, 

 with a perforation or slit near the umbo, through which pro- 

 trudes a testaceous, straight, elliptical operculum, or plug, 

 by which the shell adheres to extraneous bodies; upper 

 valve larger, concave, and entire ; ligament large, internal, 

 and cardinal. 



Anomia undulata The Waved Anomia. Plate XIV. 



fig. 2. Suborbicular, pellucid, with strong, irregular, undu- 

 lated, longitudinal striae, and some transverse concentric 

 ones ; outside yellowish dirty green ; inside fine green ; the 

 ligament perforation large, ovate; muscular impression star- 

 shaped. Two inches broad. Inhabits the British coasts. 



Genus 11 PLACUNA.— ZawwrcA. 



Generic Character. — Shell free, subequivalve, irregular, 

 very flat; hinge internal, with two short longitudinal diver- 

 gent ribs, in the form of the letter V, which fit into two 

 grooves in the opposite valve, held together by the ligament. 

 l3 



