123 
The imbricated surface and great thickness of this spe- 
cies seems at first to place it at a distance from others of 
the Genus, but there are many, both recent and fossil, 
to associate with it, and perhaps some of the characters 
may hereafter become Generic distinctions. I show a 
few more of this family on the next plate to make them 
more familiar : their external characters are sufficient to 
connect them, although we cannot always separate the 
valves as we have done in the present specimen. 
