marine Dritish Shells and Animals. | 189 
small perforation. It is thin,except about the hinge, sub-pellucid 
and brittle, but not glabrous; for by the assistance of a powerful 
lens the whole surface is observed to be minutely striated in a 
decussated order, appearing like fine shagreen: there are also 
some irregular concentric wrinkles very obvious to the naked 
eye: the margin is not regularly rounded, but in the front two 
-sub-angles are formed by the line of regular curvature becoming 
less flexuous. 
'The inside is of a singular structure about the hinge: beneath 
the beak of the upper or perforated valve the shell is very thick, 
rising on each side into a process that forms the inseparable joint 
or hinge, which firmly unites the two valves: this contrivance 
also forms a channel of communication with the aperture in the 
beak, adapted to the tube or syphon of the animal: further 
within the shell, but connected with the channel, is a depression 
which is roughened by two or three very slight longitudinal 
ridges. "The lower valve is also much. thickened at the beak, 
and rises into a transverse ridge, standing above the plane of the 
margin, in the middle of which there is a groove corresponding 
with the channel to the perforation in the beak of the other valve, 
and the sides reflect for the purpose of receiving the fangs of the 
opposite valve; and by such contrivance they are similarly and 
as firmly articulated as the joint of the claw of a crab, without 
the assistance of a connecting cartilage, of which it seems to be 
destitute: from each side of the interior part of the transverse 
ridge, a sub-arcuated compressed process or toóth projects in- 
wards nearly to the middle of the shell, their points reflecting 
and a little diverging ; at the base of each of these another simi- 
lar process, but smaller, stands erect. ‘The colour of the shell is 
pale brown, or sullied white. Length of the superior valve nine- 
| D EE a eighths 
