THE AVONIAN OF THE AVON GORGE 
159 
Spirifer lineatus (Mart.), as typified by small transversely- oval 
forms, without fold or sinus, agrees in external ornament, 
but differs in the absence of dental plates. 
Reticularia cf. reticulata, which is especially characteristic of 
the Zaphrentis-Zone, is remarkable for the reticulate under- 
layer, formed of slender, much-bent rods which fork and 
intercalate (see figure above). This species has strong 
dental plates, but the external spines appear to be single- 
barrelled in the majority of cases (although specimens which 
exhibit the normal structure very clearly occur in the same 
zone). 
Figures: Dav. Plate 13. 
Spiriferina. 
The Carboniferous members of this genus exhibit the following 
characters : — 
Shells oval with a short hinge line and a triangular area which 
is concave and often of considerable height. 
Beak pointed and curved over the area. 
Ornament : 
Strong radial pleats,^ separated by equally strong furrows. 
(The valve-intersection is consequently strongly maxil- 
la ted.) 
Mesial fold formed of a single large central pleat which is 
usually undivided (but frequently grooved along its 
middle line). 
The concentric ornament usually consists of close imbri- 
cating lamellae.^ 
The shell is strongly punctate throughout (a structure which 
is often beautifully displayed on the cast). 
Internal Septa 
In the pedicle valve, the dental plates are strongly developed, 
but do not converge and unite ; a tall septum projects 
from the middle line of the valve. 
^ In the genotype, Spiriferina rostrata of the Middle Lias, radial pleats 
and concentric lamellae are absent. 
