BIVALVIA. 285 
Generic Character. Shell transverse, inequilateral, oblong, or subrhomboidal, 
equivalve, slightly gaping at both extremities, sometimes in front ; hinge with one or 
two cardinal teeth, which are generally obsolete when full grown; muscular im- 
pressions ovate, strong, and distant; palleal impression somewhat irregular, with a 
small or moderate sinus. Ligament external. 
Animal oblong, or club-shaped ; mantle united, except where open in front for the 
passage of a digitiform foot, furnished with a byssal groove; siphons short, separated 
at their extremities ; branchial and anal orifices large, margined with cirrhi. 
Animals of this genus are generally found located in rocks, as the name imports, 
and they are often met with in those situations into which they must have entered at 
a very early age, their extrication being effected only by a fracture of the stone from 
depths of sometimes nearly six inches. It is therefore evident this aperture is formed 
by the animal itself, and as the crypts are not symmetrical, like those of the Pho/ades, 
the mechanical theory of a rotatory motion, by the rasping of the shell, will not in this 
instance satisfactorily explain the modus operandi. They appear endowed with the 
power of spinning a byssus, by which they are sometimes moored to the sides of the 
cells, and occasionally the ventral opening is of considerable size; from which 
circumstance a genus was formed by Baron Cuvier, under the name Byssomya, for the 
reception of those shells possessing this character. 
Their peculiar habits producing often great distortion, and their extraordinary 
variation, have caused much perplexity to the naturalist; the same species has, I 
believe, been placed in five different genera; and the most distinguished conchologists 
of the present day are still at variance, not only in the determination of the species to 
which the shells now found on our own coasts should be assigned, but even the generic 
limits cannot be agreed upon. A small shell from the Older Tertiaries of this 
country is in the Cabinet of Mr. Edwards, probably belonging to this genus; and 
M. Deshayes has described some species from the Paris Basin. 
1. Saxrcava RuGOSA, Pennant. Tab. XXIX, fig. 3, a—v. 
MyrtiLus Rucosus. Penn. Brit. Zool., ed. 1, vol. iv, p. 110, t. 63, fig. 72. 
—_— PHOLADIS. Miill. Zool. Dan., t. 87, figs. 1—3. 
— —_ Chemn. Conch. Cab., vol. viii, p. 154, t. 82, fig. 7395. 
Mya ByssIrERa. Otho. Fabr. Faun. Groenl., p. 408, No. 8. 
— Rustica. Broc. Conch. Foss. Subap., p. 533, t. 12, fig. 11, 1814. 
Saxrcava RuGosa. Ford. and Hanl. Brit. Moll., vol. i, p. 146, pl. 6, figs. 7, 8; and pl. r, 
fig. 6. 
— — Lyell. Trans. Geol. Soc., 2d series, vol. vi, pl. 16, fig. 7, 1839. 
—_ — J. Sow. Min. Conch., t. 466. 
-- —_ Phil. En. Moll. Sic., vol. i, p. 20, t. 3, fig. 4, 1836. 
— sTRIATA. Fl. de Bellevue. Journ. de Phys., tom. liv, p. 349, 1802. 
— GALLICANA. Desh. 2d ed. Lamarck, tom. vi, p. 162, 1835. 
