THE AVONIAN OF THE AVON GORGE 
153 
resemblance to the large finely:ribbed Chonetes which are 
usually assigned to Chonetes 'pa'pilionaeea, (Phillips) d 
The muscular scars, which include a pair of accessory 
adductors, are arranged on exactly the same plan as they are 
in Chonetes aff. comoides, although they are much less im- 
pressed (a necessary result of the diminished shell-thickness 
and convexity). Hence, there seems to be little doubt that 
Chonetes aff. comoides was derived from the earlier Chonetes 
cf. fafilionacea by convergence with Productus giganteus in 
Di. 
Chonetes cf. comoides is difficult to classify because it is as yet 
incompletely known, in spite of its great abundance. In size 
and thickness of shell, this form often surpasses the largest 
Daviesiella, but the aspect is usually markedly Chonetoid. 
Area, teeth, spines and tubuli are well developed, as also are 
the rows of pits ; the muscular scars are as yet very imper- 
fectly known. It seems probable however that Chonetes cf. 
comoides, Chonetes cf. papilionacea and Chonetes aff. comoides 
are all mutations of the same gens. (It seems possible 
that, at the birth of the gens of papihonaceous Chonetes, the 
environment was so eminently suitable that the animals were 
encouraged to build large and ponderous shells, a habit which 
they had to partially discard during Seminula-tim.Q.) 
Leptcena. 
Hinge line straight and forming the widest part of the shell. 
Pedicle valve usually horizontal and convex, with a 
dependent margin. 
Ornament consisting of fine thread-like radial ribs, crossed 
by coarse concentric wrinkles which are confined to the 
horizontal portion. 
Area narrow ; teeth prominent. 
Two large fan-shaped diductors, enclosing a narrow mesial 
adductor. 
^ This species is actually unknown since the liolotype is an imperfect 
specimen which only exhibits a portion of the interior of the brachial valve. 
M 
