CONCHIFERA. 
431 
tached by a short pedicle to extraneous marine bodies; the larger 
valve with a projecting umbo, frequently bent, and perforated at 
its summit by a round hole ; hinge with two teeth; two slender 
osseous processes arising from the disk of the smaller valve. 
This genus contains two sections:—* Shell without furrows 
or striae. ** Longitudinally furrowed. 
Terebratula cranium . —The Skull Terebratula. 
Plate LXXXVII. fig-. 1. 
Slightly ovate, ventricose ; summit of the convex valve pro¬ 
duced and blunt; with divergent striae, and the margin crenu- 
lated. Half an inch long. Inhabits the sea at the Zetland 
Islands. 
Genus 3. — ORBICULA.— Lamarck . 
Generic Character. —Suborbicular, inequivalve; without any 
visible hinge ; under valve very thin, depressed, and adhering to 
marine bodies ; upper valve subconic, vertex acute, and more or 
less elevated. 
Orbicula Norwegica. — The Norwegian Orbicula. 
Plate LXXXVII. fig. 32. 
Upper valve in the form of a depressed cone, with a produced 
and pointed summit. Inhabits the Norwegian seas. 
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.— Lamarck. 
Generic Character. —Inequivalve, suborbicular; lower 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 LXXXVII. fig. 5. 
Orbicular ; the upper valve gibbous and conical; lower valve 
flat, with three perforations. Inhabits the Indian ocean. 
