12 i MONOMYARIA. 



Genua 5.— DISCINA.— Zamarcyl. 



Generic Character. — Inequi valve, of a roundish oval shape, 

 and depressed ; valves equal in size, each provided with an 

 orbicular and central disk ; that of the upper valve not per- 

 forated, but having a mammillated projection in the centre; 

 the lower valve divided by an oblong fissure. 



Discina ostreoides. — The Oyster-like Discina. Plate 

 XIV. fig. 8; a, the upper valve; 6, the under valve. Upper 

 valve longitudinally ribbed, crossed by concentric striae. Five 

 lines long. Inhabits the Zetland and Hebridian seas. 



GewM.s 6.— BIROSTRITES.— iamarc^. 



Generic Character. — Inequi valve, with two horns ; the 

 disks of the valves elevated, conical, and unequal, obliquely 

 diverging, and nearly straight, in the form of horns; the one 

 valve enveloping the other at the base. Fossil. 



Genus 7 CALCEOLA Lamarck. 



Generic Character. — Inequivalve, triangular, turbinated, 

 flattened beneath ; the large valve hollowed like a hood, and 

 truncated obliquely at the aperture ; the smaller valve some- 

 what orbicular, and in the form of a lid, having a furrow in 

 the centre. Fossil. 



Genus 8.— RADIOLITES — Lamarck. 



Generic Character. — Shell inequivalve, striated externally, 

 the strise radiating from the umbo ; lower valve turbinated, 

 and larger; the upper convex or conical, and lid-shaped. 

 Fossil. 



Genus 9.— SPH^RULITES.— XawarcA. 



Generic Character Shell inequivalve, orbiculato-globose, 



somewhat depressed above, roughened with large subangular 

 spreading scales ; the upper valve the least, flattened, lid- 



