85 
PHOLADOMYA. G. B. Sowerby. 
Gen. Char. A transverse thin subhyaline ven- 
tricose shell ; the posterior portion short, 
rounded ; the anterior more or less elongated 
and gaping : the hinge composed of an elon- 
gated subtrigonal foveola, and a marginal 
plate in each valve with a rather short ex- 
ternal ligament ; the muscular impressions 
two, indistinct ; the sinus in the impression of 
the mantle large; the umbones approximated . 
The thin shells of this genus have generally several 
oblique ribs or rows of gentle elevations upon the sur- 
face, with corresponding hollows within. The inner 
surface is pearly. 
Until the discovery of a recent species from the 
island of Tortola induced my brother to establish this 
extremely natural genus, the fossils belonging to it 
were scattered in various genera, to none of which they 
had any good claim to be united. 
