38 



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, PI. I., and fig. 10, 

 PI. II.). 



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 

 ligament, 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 ''^alves 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 



