BIVALVIA. 277 
Mya.* Linneus, 1747. 
Mya, Lamk. 1801. 
SpHENrA (sp.) Turt. 1822. 
Generic Character. Shell transverse, equivalve, subequilateral, more or less of an 
ovate or oblong form, gaping at the siphonal extremity; externally striated or 
furrowed by rough or irregular lines of growth; in the recent state covered by an 
epidermis. Hinge composed of a large, projecting, spoon-shaped tooth or process in 
the left valve, on which is placed the ligament, with a corresponding depression or 
socket beneath the umbo in the right valve. Impressions of the adductor muscles 
near the extremities, with a deeply sinuated line in the mantle-mark. 
Animal of the form of the shell, with the mantle closed except in front for the 
emission of a small tongue-shaped foot. Tubes long, covered with a strong case-like 
coriaceous epidermis, separated at their extremities, with fimbriated orifices. 
This genus is more especially characterised by the peculiar form of the hinge, 
which distinguishes it from all other Bivalves, and as now restricted contains but very 
few species in the recent state. They are confined to the colder regions of the globe, 
where they live buried in mud or sand, sometimes to the depth of a foot, in an erect 
position, with the siphonal extremity upwards, their tubes extending into the 
water ; they have a vertical range from low-water-mark to upwards of 150 fathoms. 
Dead shells have been obtained from even greater depths, but it is no proof in such 
case of animals having a very extensive vertical range: Dr. Sutherland has shown, in 
regard to Arctic species, they are often transported by icebergs into very deep water. 
The secondary fossils figured under this generic name have no relationship 
whatever ; but a small species has been obtained from the upper marine of the Older 
Tertiary Periods that may be referred to this genus, showing then, as now, its some- 
what estuary character, or its tendency to approach the regions of fresh-water. 
1. Mya truncata, Linneus. Tab. XXIII, fig. 1, a—f 
Mya rruneata. Linn. Syst. Nat., ed. 12, p. 1112, No. 26, 1767. 
oo _— Gould. Invert. Massach., p. 42, 1841. 
— — Miller. Ind. Moll. Groenl., p. 21, 1842. 
— — Dekay. Nat. Hist. New York Moll., p. 240, pl. 29, fig. 289, 1843. 
—_ — Phil. En. Moll. Sic., vol. ii, p. 6, 1844 (fossil). 
a = Lovén. Ind. Moll. Scand., p. 49, 1846. 
— — Forb. and Hanl. Hist. Brit. Moll., vol. i, p. 163, pl. 10, figs. 1, 2; and 
Animal, pl. 4, fig. 1, 1848. 
* Etym. piaé, a kind of shell-fish. 
