BIVALVIA. 277 



Mya.* LinncBiis, 1747. 



Mya, Lamfc. 1801. 

 Sphenia (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 fi'om 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, Linnceus. Tab. XXIII, fig. 1, a—f. 



Mya truncata. Linn. Syst. Nat., ed. 12, p. 1112, No. 26, 1/67. 



— — Gould. Invert. Massach., p. 42, 1841. 



— — Moller. Ind. Moll. Groenl., p. 21, 1842. 



— — Bekmj. Nat. Hist. New York Moll., p. 240, pi. 29, fig. 289, 1843. 



— — Phil. En. Moll. Sic, vol. ii, p. 6, 1844 (fossil). 



— — Loven. Ind. Moll. Scand., p. 49, 1846. 



— — Forh. and Hani. Hist. Brit. Moll., vol. i, p. 16;5, pi. 10, figs. 1, 2; and 



Animal, pi. h, fig. 1, 1848. 



* Etym. fxiin^, a kind of shell-fish. 



