182 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
grooves are very prominent, and, in a full grown shell, 3 
to 6 such can readily be observed which probably mark the 
limits of as many successive years of growth. Occasion- 
ally these grooves are very distinct. Looked at from the 
outside the margin is nearly even; on the inside it is 
deeply notched, the depressions corresponding in position 
with the ribs on the outside (see fig. 1, Pl. I., and fig. 10, 
Plan): 
The hinge line is gently curved, and the concavity 
of the shell is continued dorsally beyond the hinge into 
the umbo; along this hinge line is a series of double teeth 
on each valve which interlock when the valves are closed. 
On the right valve the central cardinal tooth has the form 
of a deep depression with sharp cusps arranged anteriorly 
and posteriorly. On the left valve this arrangement is 
reversed, there being a single median cusp with depressions 
in front and behind; similarly on the right valve each of 
the two lateral teeth (which are really anterior and pos- 
terior) consists of a prominent ventral and a smaller 
dorsal cusp with an elongated depression between. On 
the left valve this arrangement is reversed. The hinge 
hgament, which represents the dorsal uncalcified portion 
of the shell, lies posterior to the umbones and is external; 
it is hollow and arched, underneath it the dorsal margin 
of the valves do not come into contact when the shell is 
closed, and a median glandular fold of the mantle projects 
up into the cavity beneath the arch of the ligament, and is 
in contact with the internal surface of the latter. The 
ligament is very elastic and serves for the divarication of 
the valves when the adductor muscles are relaxed. 
There is little pigmentation on the shell except at the 
posterior margin, where, particularly on its internal 
surface, it is tinted a chocolate brown or green. This 
pigmentation may extend on to the scar of attachment 
