160 
THE AVONIAN OF THE AVON GORGE 
The essential characters of the genus are : — 
(1) Pointed incurved beak and concave area. 
(2) Strongly punctate shell. 
(3) Prominent mesial septum in the pedicle valve. 
Figures: Dav. Plate 7, figs. 37 to 55. 
Syringothyris. 
Hinge- line almost, or quite, as wide as the shell. 
Shell dispersedly punctate. 
Brachial-valve transverse. 
Mesial fold usually composed of a single, large undivided ^ 
pleat. 
Simple radial ribs on the flanks. 
A mesial septum usually developed. 
Pedicle-valve semiconical, the beak forming the apex 
and not curved over the area. 
Area very large, triangular, and usually flattened : sinus 
deep, semiconical, and usually devoid of ribs.^ 
Ribs on flanks numerous, but usually much flattened. 
Internal characters of the pedicle valve : — 
Mesial septum and dental plates strongly developed. A 
transverse sheet stretches across the delthyrium, just 
within the aperture, and, along the middle fine of the 
inner surface of this sheet, a split-tube (the ‘ syrinx ’) 
is formed by the growth of two curved plates towards 
each other. 
The essential character of Syringothyris is the presence of 
a syrinx, and it is only by demonstrating the presence of this 
structure that specimens can be definitely separated from 
Spiriferina, a genus in which the area may become large and 
flat, and in which the delthyrium is occasionally partly closed 
by a rostral callus. 
(Syringothyris cf. laminosa is an instance of a Spiriferid 
which is not yet definitely separated from Spiriferina!^) 
1 Tn Syringothyris distans, both fold and sinus are irregularly ribbed. 
- See Q.J.G.S. vol. 61, p. 301. 
