108 MONOMYAIRA. 



TRIBE II. RUDISTA. 



Animal unknown, as are also the ligament and hinge ; shell 

 with very unequal valves, and having no distinct umbones. 



Genus 4 CRANIA. — Bruguiere. 



Generic Character. — Inequivalve, suborbicular ; under valve 

 flattened, fixed below, and perforated in its lower surface by 

 three unequal oblique holes ; upper valve very convex ; provided 

 interiorly with two projecting callosities. 



Crania personata. — The Masked Crania. Plate XIV. 

 fig. 5. Orbicular ; the upper valve gibbous and conical ; lower 

 valve flat, with three perforations. Inhabits the Indian 

 ocean. 



Genus 5.--DIS CIN A.— Zawjcrcy^. 



Generic Character. — Inequivalve, of a round oval shape, 

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

 bicular and central disk ; that of the upper valve not perforated, 

 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 ; b, the under valve. Upper 

 valve longitudinally ribbed, crossed by concentric striae. Five 

 lines long. Inhabits the Zetland seas. 



Genus 6.— BIROSTRITES— Zawiarc^. 



Generic Character. — Inequivalve, with two horns ; the disks 

 of the valves elevated, conical, and unequal, oblique, diverging, 

 and suberect, somewhat horn-shaped ; the one valve enveloping 

 the other at the base. Fossil. 



Getius 7.— CALCEOL A.— Lamarck. 

 Generic Character. — Inequivalve, triangular, turbinated, flat- 



