12 i MONOMYARIA.. 



Genua 5.— DlSCm A.— Lamarck. 



Generic Character. — Inequi valve, of a roundisli 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; h, the under valve. Upper 

 valve longitudinally ribbed, crossed by concentric strise. Five 

 lines long. Inhabits the Zetland and Hebridian seas. 



Genua 6.— BIROSTRITES.— ZamarcA. 



Generic Character. — Inequivalve, 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 striae radiating from the umbo ; lower valve turbinated, 

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

 Fossil. 



Genus 9— SPHiERULITES.— XawiarcA. 



Generic Character, — Shell inequivalve, orbiculato-globose, 

 somewhat depressed above, roughened with large subangular 

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



