14 



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 

 groove is formed, knoAvn 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 majoritv 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. maximus. 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 

 umbo in such a way that when the shell is closed the most 

 dorsal point is formed by the convex valve which is 

 slightly folded over to join the upper flat valve. 



The inner faces of the valves are marked by impres- 

 sions indicating the attachments of the various muscles. 



The Pallial Line is a scar marking the attachment of 

 the numerous retractor muscles of the mantle edsre. 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 fPl. I., 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. 



