12G 
PLAGIOSTOMA. 
Each valve lias from twenty-live to thirty rounded 
costa2, formed by intervening furrows, that radiate 
from the beaks to the margin ; these are decus- 
sated by fine transverse striae, and the lines of in- 
crease ; the inner surface of the shell is also marked 
with corresponding impressions. The lower valve 
is most convex, and has the line of the hinge 
straight ; the upper valve is spinous, rather de- 
pressed, and contains the angular sinus, by which 
tlie shells of this genus are characterised. 
The spines arise from the ribs, but without any 
regularity, except that they are more numerous at 
the sides, than in the centre. They vary from 
fifteen to twenty in number, and are from half an 
inch, to two inches and a half in length ; each spine 
has a groove on the under, and a corresponding 
ridge on the upper surface. They generally pro- 
ject from the shell, but in some instances lie close 
on the surface. 
The beaks are convex, and incurved ; the ears 
small, and even ; the margin neatly denticulated. 
A specimen cleared from the chalk, exhibited no 
muscular impression. 
There are several varieties of this species, of 
which the following are the most remarkable : — 
V^ar. a. With both valves gibbous, and but few 
spines. 
h. Valves depressed, spines numerous. 
c. Valves gibbous, ribs regularly convex 
and even. 
d. , ribs channelled near tlie 
front. 
This shell is one of the most common ])roductions 
