154 
THE AVONIAN OF THE AVON GORGE 
Brachial valve: horizontal portion flattened, and 
usually concave (but convex in a common D. species). 
Cardinal process and mesial septum well developed. 
Muscular scars (two pairs of adductors) deeply impressed 
and circumscribed by strong ridges. 
[Broad, ribbon-like, vascular (?) markings start near the 
line of geniculation, and run down, over the dependent 
portion of the valve.] 
Figures: Dav. Plate 28 (under the generic name Stro- 
fhomena). 
Orthothetes . 
Hinge line straight, and usually forming the widest part of 
the shell. 
Ornament consisting of sharp erect, thread-like radial ribs 
which are conspicuously periodic in strength and length 
(usually at least triserial). 
Pedicle valve flattened or concave when viewed as a 
whole, but the beak region is usually convex. 
Area usually narrow, with a central delthyrium. 
Teeth strong. 
No mesial septum. 
The diductor-scars form a continuous fan-shaped area in the 
centre of which is enclosed the small inconspicuous 
adductor-scar. 
Brachial valve convex (and often gepiculate) when 
viewed as a whole, but the beak region is usually flattened- 
and often concave. 
The cardinal process combines with the crural plates to 
form a thick, transverse projection from the middle of 
the hinge-line. 
The muscular area (adductors) is fan shaped and bisected 
by a low mesial ridge. 
Figures: Dav. Plate 26 (under the generic name Strepto- 
rhynchus). 
