MYACIDA. 137 
Famity MYACID A. 
Shell thick, strong and opaque; left valve with a spatulate 
scartilage-process, gaping posteriorly ; pallial line sinuated ; epi- 
dermis wrinkled. Structure more or less distinctly cellular, 
with dark nuclei near the outer surface ; cartilage-process com- 
posed of radiated cells. 
Animal with the mantle almost entirely closed; pedal aper- 
ture and foot small; siphons united, partly or wholly retractile ; 
branchiz two on each side, elongated. 
Mya, Linn., 1758. 
Htym.—Myazx (-acis), a mussel (Pliny). Gaper. 
Distr.—3 sp. Northern seas. Fossil, 17 sp. Pliocene—; 
United States, Britain, Sicily, 
Shell oblong, inequivalve, gaping at the ends; left valve 
smallest, with a large flattened cartilage-process; pallial sinus 
large. Most of the fossil ‘* Myas” have an external ligament, 
and are related either to Panopzxa or Pholadomya. 
Animal with a small straight linguiform foot ; siphons com- 
bined, covered with epidermis, partially retractile; orifices 
fringed, the branchial opening with an inner series of large 
tentacular filaments; gills not prolonged into the siphon; palpi 
elongated, free. 
The Myas frequent soft bottoms, especially the sandy and 
gravelly mud of river-mouths; they range from low-water to 25 
fathoms, rarely to 100 or 145 fathoms. JM. arenaria (evi, 19,20) 
burrows a foot deep; this species and MW. truncata (evi, 18) are 
found throughout the northern and Arctic seas, from Ochotsk 
and Sitka to the Russian Icy-sea, the Baltic, British coast and 
northern United States; in the Mediterranean they are only 
found fossil. They are eaten in Zetland and North America, 
and are excellent articles of food. In Greenland they are 
sought after by the walrus, the Arctic fox, and birds. (0. 
Fabricius.) 
Puatyopon, Conrad, 1837. 
Distr.—P. cancellata, Conr. (evi, 28). California. 
Shell ventricose, with concentric, undulating striz, and a small 
groove from the apex to the ventral margin; posterior side short, 
radiately striated, spoon-shaped cardinal process dilated and 
biemarginated. Siphonal orifices furnished with four valvular 
testaceous appendages, which close them. 
TuaoniA, Gray, 1842. 
Syn.—Le Tugon, Adanson. 
Disir.—6 sp. West Coast of Africa. Fossil. Miocene; Dax, 
and the Morea. TZ’. anatina, Gmel. (cvi, 21, 22). 
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