GENUS RHYNCHONELLA — STENOCISMA. 
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Genus or Subgenus Stenocisma (Conrad—Hall). 
Shells subtriangular, ovoid or subglobose; hinge-line short : beak of 
ventral valve extended, attenuate or more or less arcuate and appres- 
sed against the opposite valve. A mesial sinus and fold on the ventral 
and dorsal valves respectively. Surface plicated with simple or rarely 
bifurcating plications. 
The valves are articulated by teeth and sockets. A strong median sep¬ 
tum in the dorsal valve of many of the species, which becomes thick¬ 
ened in its upper part, with a shallow triangular pit in the centre, and 
the crura supported on each side : dental sockets crenulate. The'teeth are 
strong, with lamellae extending to the bottom of the valve, limiting the 
rostral cavity, and sometimes partially surrounding the muscular area. 
In casts of the ventral valve, the cavities made by the diverging 
lamellae are very distinct; and in the dorsal valve, the place of the sep¬ 
tum is strongly marked, the space being wide above, and within this 
cavity at the upper part there is a small angular elevation of greater or 
less extent, which originally filled a depression in the upper part of the 
septum. 
I would by no means include all the palaeozoic so-called Rhynchonellae under this 
genus, for I am satisfied that there are plicated shells of similar form which have 
a different internal structure ; but it is absolutely necessary that this structure be 
known, before we can make a proper disposition of the species. 
Rliynclionella (Stenocisma) tetliys. 
PLATE LIY. 
? Rhynchonella tdhys : Billings, Canadian Journal, p. 270. 1860. 
Shell subtrigonal ovate, usually wider than long, sometimes length and 
width equal. 
Ventral valve depressed convex in the upper part, nearly flat at the 
sides : mesial sinus beginning above the middle, moderately depressed 
in young shells, and in older ones becoming deep in front and abruptly 
curving towards the dorsal valve. 
