224 CONCHIFERA. 
called the pallial impression, or that which marks 
the attachment of the mantle. On the upper side 
VALVE OF CYTHERZA. 
is seen the hinge, an apparatus of shelly teetli 
alternating with cavities, which fit, with the most 
accurate precision, corresponding cavities and 
teeth in the opposite valve. ‘The very summit of 
the shell, which often, as in this instance, pro- 
jects into a blunt point, bending towards one side, 
is termed the beak or umbo; beneath this, on the 
anterior side, or that towards which it turns, there 
isa depressed space, often very conspicuous, termed 
the lunule; while on the hinder side of the wmbo 
is spread the dense elastic leathery igament. The 
lunule, umbo, and ligament, constitute the dorsal or 
superwor border, the lower side (in the figure) is the 
ventral or inferior border; a line drawn from one 
to the other of these measures the length of the 
shell, the width being, of course, at right angles to 
this, while the thickness is determined by a line from 
the centre of one valve to that of the opposite. 
