191 
UNIO. cordiformis. 
TAB. DXCV.—jig. 1. 
Spec. Cuar. Heart-shaped, posteriorly pointed ; 
umbones inflated. 
A ruicx shell, whose length, width, and the united depth 
of the valves, are nearly equal. The anterior side is very 
small. 
This is the rarest shell that occurs in the beds of Sand- 
stone at Tilgate Forest, subordinate to the Weald Clay 
of Sussex. 
We are under obligations to G. Mantell, Esq. for the 
loan or possession of the specimens figured in this and 
the preceding plates. 
Much diversity of opinion exists regarding the pro- 
priety of arranging the shells given in these two tables in 
the genus Unio. They appear to be confined to certain 
Sandstone beds included in the Weald Clay ; a formation 
that contains very few species of shells excepting oysters, 
and these referable to only three genera, Paludina, Cy- 
clas, and Unio. In most cases the shells are firmly united 
to the stone, and only casts can in general be extracted, 
which renders it difficult to observe those parts by which 
the genera are clearly indicated, and lay a foundation for 
doubt. The genera above assumed are composed of fresh- 
water shells; and from this it has been concluded that the 
formation they are found in is of freshwater origin, pro- 
