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THE AVONIAN OF THE AVON GORGE 
Ghoyieii-Productus. 
The forms included here only differ from Prod, giganteus in 
the possession of : — 
( 1 ) Narrow area, 
(2) Well developed teeth, 
(3) Tubuli which perforate the shell immediately below 
the surface of the area. 
[The row of spines along the cardinal edge occurs also 
in typical specimens of Prod, corrugatus, Prod, hemisphericus 
and Prod, giganteus.^ 
In Choneti-Productus Chonetoid convergence is clearly indi- 
cated, but this is rather the expression of instability in the 
species than of true mutational variation, for, among a large 
number of specimens of Prod, giganteus collected from the same 
bed and agreeing in all other characters, Chonetoid converg- 
ence is only exhibited in a small number. 
This Chonetoid-tendency has only been remarked in speci- 
mens from D 2 , and it is synchronous with the Clisiophylloid- 
tendency which is exhibited by certain specimens of Lithostro- 
tion Martini. 
Chonetes. 
Valves transverse, flattened. 
Hinge line straight, forming the widest part of the shell. 
Pedicle valve shallow-convex. 
