162 
THE AVONIAN OF THE AVON GORGE 
section. I shall consequently subdivide the group into two 
genera only, Aihyris and Seminula, based entirely upon the 
nature of the external ornament. 
Aihyris, 
Suture line with valve 
Actinoconchus. Clioihyris. 
Expansions magnified. 
Here I include all Athyrids in which the concentric orna- 
ment consists of imbricating expansions. The true nature of 
the expansion is usually difficult to settle, since it is only in 
exceptionally-weathered specimens that the structure is 
obvious and an excess of weathering, by destroying part of 
the expansion, leads to an erroneous conception of its true 
nature. 
Two distinct types of expansion are figured above : — 
(1) The Actinoconchus-tj^Q in which the lamellae consist of 
flattened tubes, united by a web. This structure is typically 
developed in certain specimens which are referred to Aihyris 
planosulcata. When the expansion has been laid bare by a 
lucky fracture, the tubes appear as ribs upon a broad lamella ; 
but when a specimen has been subjected to weathering, the 
web is removed faster than the ribs and the expansion conse- 
quently appears to be fringed. 
(2) The Clioihyris-tji^e, in which the expansion consists of 
close-set, disunited, flattened tubular spines which spring 
from a narrow basal lamella. This structure is characteristic 
of Aihyris Royssii. 
Figures: Dav. Plates 16 and 18. 
