176 
easily; in this, however, the thinness of the beak is such f 
that it is not often preserved entire. Plagiostoma (from 
nhayiog, oblique, and $opta 9 mouth) is the trivial name given 
by Luid to my gigantea, from which it should appear that 
he had not overlooked the external form of the back, with 
the triangular opening for the cartilage of attachment. 
The species of this Genus being apparently but little under- 
stood, I have taken some pains to examine them, and the 
tenderness of the beak or hinge end, which is extremely 
thin and brittle, and commonly imperfect, rendering this 
difficult, I am the more pleased that I have thus succeeded. 
PLAGIOSTOMA gigantea. 
TAB, LXXVII, 
Spec. Char. Smooth, depressed,, deltoid with the 
posterior side rounded into the front; ears 
small, anterior one longest, placed in a large 
broad and straight furrow; beaks pointed; 
surface obscurely marked with diverging strise. 
xVxthqugh this shell is marked with lines of growth and 
diverging striae, yet its general aspect is smooth ; often the 
lines are very obscure, as if worn away, and the surface 
appears polished : it is deepest towards the straight anterior 
side; its width is about four-fifths of its length, and its 
greatest depth about one-fifth. 
This species is found in great variety in the Bath Lyas 
or Foetid Limestone, both the blue and white, the speci- 
mens partake of the colour of the stone in which they are 
imbedded. They are often of large dimensions: I have 
one nine inches in diameter, and another very large one 
