104 DEVONIAN FOSSILS 
sions of different parts of the same biological-specimen, they are 
collectively regarded as the holotype. Paratype consisting of the 
steinkern of a dorsal valve (specimen 27,235) from locality G 25. 
It is to be noted that the paratype comes from a different locality 
from that of the holotype, but the two localities are close to one 
another both geographically and stratigraphically. A large part 
of a dorsal valve is also present in the material from G 22 (speci- 
men 27,179). 
Description of Holotype. Ventral valve outline subcircular. 
Valve flexed so as to be a little convex near the umbo and a 
little concave for most of the remainder of the shell, Hingeline 
shorter than greatest width. The broken margin precludes precise 
measurement, but the shell was 6-5-7 em. long and 7-5-8 em. wide. 
Ornamentation of costellae radially disposed except that those 
near the hingeline are bent back to meet it. On the umbonal half 
of the shell the ornament looks comparatively disperse, while in 
the marginal area it looks closely packed. On the umbonal half the 
costellae appear to alternate in size, while in the marginal half 
they appear to be of equal size. This general appearance is due, 
first of all, to the presence of primary costellae which alternate 
in the umbonal half of the shell with secondary costellae. The 
primary costellae can be traced right to the umbo, where the 
secondary costellae are so fine that they cannot be traced without 
doubt. The secondary costellae increase in size until they equal 
the primary ones. About a third of the way to the anterior margin, 
tertiary costellae are intercalated, and by half way down the shell 
they become a noticeable part of the ornament through increase in 
size. 
Thus in the marginal half of the shell, primary, secondary and 
tertiary costellae, having reached equal size, impart a regular and 
fine appearance to the ornament. Towards the margin, further 
intercalations and some bifurcations cause the regularity of the 
ornament to be maintained. Concentric ornamentation is also 
present in the form of fine lines which in the centre were counted 
as 28 per cm., but on the sides of the shell are still finer, because 
the shell has to grow faster anteriorly than laterally in order to 
maintain its proportions. Yet a third type of ornamentation is 
present in the form of concentric rugae, or rather furrows. The 
' most marked and continuous of these are where the more disperse 
ornament of the posterior end of the shell is replaced by the more 
regular ornament of the anterior end. The ornament of the dorsal 
valve is similar to that of the ventral valve. This description has 
been made from a plasticine impression of the external mould of 
the holotype. 
