THE AVONIAN OF THE AVON GORGE 
155 
Derby a. 
Cast of pedicle valve (beak region only), showing impression of mesial 
septum. 
Back-and-front section, showing mesial septum in pedicle valve. 
This genus is separated from the normal type of Orthothetes by 
the following characters : 
Strong mesial septum in the pedicle valve. 
[Strongly convex brachial valve and flattened pedicle valve.] 
Large area whose surface is usually bent out of one plane. 
Delthyrium usually twisted to one side. 
Surface of valves very irregular in curvature and often 
undulated. 
Figures; Dav. Plate 27, figs. 1, 2, 3, 4 {Streptorhynchus 
crenistria, var. senilis). 
Schizophoria and Rhipidomella. 
Characters common to both genera : — 
Valves biconvex. 
