114 DIMYARIA. SOLEN. 
internal, large, and fixed in the cavity of the tooth in one 
valve, and to a large subumbonal cicatrix in the other. 
Mya lata. Plate X. fig. 22. 
The Myz are easily distinguished, by the large upright tooth in one valve 
only; they are marine shells, and inhabit the seas of the Northern Hemi- 
sphere. 
They are found in a fossil state in the Crag, the Blue Marls of France, and 
also in the Cretaceous and Oolitic group of rocks. 
TRIBE II.—SOLENIDES. 
Shell transversely elongated, destitute of accessory pieces, 
and gaping only at the lateral extremities ; ligament external, 
Genus LII. — PANOPAA. — Mesnard. 
Generic Character. — Equivalve, oval, inequilateral, 
gaping unequally at both extremities; hinge with an acute 
primary tooth in each valve, and a large callosity near the 
umbones, supporting the ligament; two distant, oval, 
muscular impressions, pallial impression with a large sinus ; 
ligament large, external, adhering to an ample prominent 
fulcrum. 
Panopea bivone. Plate XI. fig. 10. 
The shells of this genus bear a general resemblance to those of Mya, but 
differ in having a sharp tooth, and external ligament, instead of the broad, 
spoon-like process in the hinge of Mya. 
They inhabit the ocean, and one species has been dredged off Scarborough. 
In a fossil state they are met with in the Blue Marls of France, the super- 
eretaceous rocks of Bordeaux and Dax, and in the Greensand. 
Genus LITT. — SOLEN. — Linneus. 
Generic Character.— Shell equivalve, transversely elon- 
gated, subcylindrical, prodigiously inequilateral, umbones 
nearly terminal, situate close to the anterior side, and gaping 
widely at both extremities; truncated, or subtruncated, 
sometimes rounded; hinge linear, with several small cardinal 
teeth, various in form, often acute and recurved ; lateral 
teeth somewhat elongated and crooked ; muscular impres- 
sions distant, tongue-shaped, the anterior one joined a little 
behind the umbones; the posterior one irregular and sub- 
oval; pallial impression elongated, straight and bifurcated 
behind; ligament long and exterior; external surface covered 
with a thick horny epidermis. 
_ Solen fragilis. Plate X. fig. 19. 
The above generic character embraces only those shells whose general form 
