490 MANUAL OF THE MOLLUSCA. 
fathoms. MM. arenaria burrows a foot deep; this speciesan1 M. 
truncata are found throughout the northern and Arctic seas, 
from Ochotsk and Sitka to the Russian Ice-meer, the Baltic, 
and British coast; 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. (O. Fabricius.) 
Fossil, 17 species. Pliocene—. United States, Britain, Sicily. 
Most of the fossil ‘‘Myas” have an external ligament, and are 
related either to Panopea or Pholadomya. 
CoRBULA, Bruguiére. 
Etymology, corbula, a little basket. 
Type, OC. sulcata, Pl. XXIIT., Fig. 2. 
Synonyms, Hrodona, Daud. (=Pacyodon, Beck.) Agina, 
Turt. 
Shell thick, inequivalve, gibbose, closed, produced posteriorly ; 
right valve with a prominent tooth in front of the cartilage pit; 
left valve smaller, with a projecting cartilage process; pallial 
sinus slight; pedal scars distinct from the adductor impressions. 
Animal with very short, united siphons; orifices fringed; 
anal valve tubular; foot thick and pointed; palpi moderate ; 
gills 2 on each side, obscurely striated. 
Distribution, 66 species. United States, Norway, Britain, 
Mediterranean, West Africa, China. Inhabits sandy bottoms; 
lower laminarian zone—80 fathoms. 
Fossil, 120 species. Inferior Oolite—. United States, Europe, 
India. 
The external shell-layer consists of fusiform cells; the inner 
is homogeneous and adheres so slightly to the outer layer, that — 
it is very frequently detached in fossil specimens. Corbulomya, 
Nyst (C. complanata, Sby.), Crag., Britain. 
Sub-genera, Potamomya, J. Sowerby. P. gregaria, Eocene, 
Isle of Wight. Cartilage process broad and spatulate, received 
between two obscure teeth in the right valve. The estuary 
Corbule differ very little from the marine species. P. labiata 
(Azara, D’Orbigny), Pl. X XIII., Fig. 3, lives buried in the 
mud of the River Plata, but not above Buenos Ayres, and con- 
sequently in water which is very little influenced by the 
superficial ebb of the river. The same species is found in banks 
widely dispersed over the Pampas near San Pedro, and many 
places in the Argentine Republic, five yards above the river 
Parana. (Darwin.) 
Sphenia, Turt. S. Binghami, Pl. XXIII., Fig. 4. Shell 
