98 DIMY ARIA. 7 PULLASTRA. 
Cardiola fibrosa. Plate XI. fig. 6. 
The shells of this genus are highly characteristic of the lower members of the 
upper Silurian Rock, and are spread over a wide extent of country. 
No recent species have been met with. 
Genus XXII. — MYOCONCHA. — Sowerby. 
Generic Character. — Bivalve, equivalve, oblique, sides 
very unequal; hinge with an elongated oblique tooth in the 
left valve, and provided with an external ligament; beaks 
placed close to the posterior extremity ; destitute of a sinus 
in the impression of the mantle. 
Myoconcha crassa. Plate VII. fig. 34. 
In its general contour this shell resembles those of Modiola; and in the 
form of the hinge is somewhat allied to the genus Crassina ; and to the Con- 
che generally ; but is, however, destitute of lateral teeth, and deficient in the 
posterior lobe ; the posterior muscular impression is strongly indented. There 
is but one species known, and it occurs in a fossil state at Dundry, near 
Bristol. 
TRIBE V.—CONCHACEA. 
Shells with three primary teeth at least in one valve, and 
the other generally with the same number, but in some 
instances fewer. 
SUB-DIVISION I.—MARINE.—Generally destitute of lateral teeth. 
Genus XXIII. — VENERICARDIA. — Lamarck. 
Generic Character. — Shell equivalve, inequilateral, sub- 
orbicular, the surface generally with longitudinal radiating 
ribs or furrows ; two oblique primary teeth, directed to the 
same side. 
Venericardia planicosta. Plate VII. fig. 26. Found 
fossil in the London Clay, Hampshire. It is also met with 
in the neighbourhood of Paris, at Piedmont, and Florence. 
Several other species occur in the London Clay, and Crag 
of England, as also in the Supercretaceous rocks of Dax 
and Bordeaux. 
The Venericardiz are marine shells; one recent species only is known, which 
is peculiar to the seas of New Holland. 
Genus XXIV.— PULLASTRA.— Sowerby. 
Generic Character.— Shell equivalve, transverse, inequi- 
lateral, the anterior side being the shorter; three diverging 
cardinal teeth in both valves, situate near to each other, and 
generally with a notched or cleft termination ; and ina few 
