THE AVONIAN OF THE AVON GORGE 
161 ‘ 
Figures: Dav. Plate 8, 
Cyrtina. 
The Carboniferous members of this genus exhibit the follow- 
ing characters : — 
Form: either Syringothyris-\\kQ, as in Cyrtina seftosa, or 
Sfiriferina-Y^Q, as in Cyrtina carhonaria. 
The area is large and triangular, with a large central del- 
thyrium. 
Ornament: 
The mesial fold is scarcely differentiated from the flanks, 
and the same type of ribbing extends over the whole shell. 
The ribs are usually thick and frequently forked. 
Internal structure of the pedicle valve: 
Mesial septum and dental plates are strongly developed. 
The dental plates converge and unite with each other, and 
with the mesial septum, to form a prominent s'pondylium. 
The shell structure is strongly punctate. 
The essential characters of the genus are the prominent 
spondyhum and the strongly punctate shell. 
Figures: Dav. Plates 14 and 15. 
The Athyrids 
The Carboniferous species of this group are distinguished by 
the following characters : — 
Curved hinge line and no area. 
Perforated beak. 
Internal spires, directed to the sides. 
Shell structure non-punctate. 
Notwithstanding the large amount of work which has been 
done upon the members of this group, the divisions into which 
it has been separated cannot as yet be regarded as entirely satis- 
factory. Since also the essential distinctions between the 
divisions rest upon the structural characters of the spires, a 
critical discussion of their value would be entirely out of place 
in an elementary introduction such as is attempted in this 
