72 
Proceedings of tlie Eoyal Society of Edinburgh. [Sess. 
their upper margin. The posterior median plate seems fused with the 
valves ; its upper notch is deep, but not surrounded by a horse-shoe-shaped 
inner profile. 
thickening. Ridge {r.) triangular, short, wide, depressed. The gutter-like 
apodema is deep; it ends in a blind diverticulum (cul-de-sac = a.), from the 
posterior surface of which rises a second remarkable appendage. This 
