MONOMYAIRA. 109 



tened beneath ; the large valve hollowed like a hood, and trun- 

 cated obliquely at the aperture; the smaller valve somewhat 

 orbicular, and in the form of a lid, having a furrow in the centre. 

 Fossil. 



Genus a— RADIOLITES.— X«.m«ra-. 



Generic Character. — Inequivalve, and interiorly striated, ra- 

 diating from the umbo ; lower valve turbinated, and the largest ; 

 the upper one convex or conical, and lid-shaped. Fossil. 



Genus 9. — SPHiERULITES.— La-?2arcA. 



Generic Character. — Inequivalve, suborbicular-globose, some- 

 what depressed above, roughened with large subangular horizon- 

 tal scales ; the upper valve the least, flattened, lid-shaped, with 

 two unequal tubercles within ; under valve subventricose, with 

 the scales radiating beyond the margin, and forming a ridge ; 

 insides partly striated ; hinge unknown. 



Section II. — Ligament not marginal, inclosed in a short 

 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 — KliiOMlA.—LinncBus. 



Generic Character. — Shell inequivalve, irregular, operculated, 

 adhering by the operculum ; under valve flattened, with a large 

 circular or ovate perforation near the umbo, through which pro- 

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

 which the shell adheres to extraneous bodies ; upper valve larg- 

 er, concave, and entire ; ligament large, internal, and cardinal. 

 K 



