346 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIELY. 
margins form acute angles, and that of the right valve is 
reflected so that the anterior left auricle overlaps it at this 
point. The valve is also depressed here slightly, so that a 
eroove is formed, known as the Byssal Notch, and it is 
deeper in young forms than in the adult. Since the foot 
is situated so near the hinge line, it is probable that the 
groove is due to its presence, because the valves would 
otherwise have to open much wider for the protrusion of 
foot and byssus than is the case in the majority of 
Pelecypods where the foot is protruded ventrally. This 
would also account for the greater depth in younger forms 
and absence in the adult P. mavimus. At the base of the 
byssal notch are three tooth-like processes, the function of 
which is unknown. The hinge line is almost level, but in 
the convex valve it rises slightly on either side of the 
wnbo in such a way that when the shell is closed the most 
dorsal poit is formed by the convex valve which 1s 
shghtly folded over to join the upper flat valve. 
The inner faces of the valves are marked by umpres- 
sions indicating the attachments of the various muscles. 
The Pallial Line isa scar marking the attachment of 
the numerous retractor muscles of the mantle edge. It 
is a sinuous line extending without break or indentation 
(owing to the absence of siphons and their retractor 
muscles) almost parallel to the shell margin, at a distance 
of about one and a half inches from it, at the ventral 
border (Pl. L., fig. E). 
The adductor impression is larger on the flat upper 
valve than on the lower convex one. This impression is, 
moreover, situated nearer the ventral margin of the shell 
on the left valve than on the right, owing to the oblique 
track of the muscle fibres. The single retractor of the 
foot is attached to the left valve, but its impression forms 
part of the adductor impression. 
