16 JURASSIC RHYNCHONELLIDS 
been the cause of some of the misunderstanding. The first full description of the 
septalium showing its appearance in transverse section was given by Muir-Wood 
(1934) when she chose as her example the zeilleriid Digonella digona (Sowerby), 
despite the fact that it had originally been defined with reference to a rhynchonellid. 
The septalium found in D. digona is relatively shallow and supported by a high 
septum. These features, coupled with the flattened nature of the brachial valve, 
result in the appearance, in transverse section, of a septalium bearing little resem- 
blance to the same structure as seen in most Mesozoic rhynchonellids. As stated 
above, the septalium in zeilleriids consists of a well developed, shallow trough lying 
between the hinge plates and supported for some distance by a high median septum. 
In contrast to this, the septalium in Septaliphoria arduennensis is only developed at 
the extreme posterior end of the valve and is either supported by a very low septum 
or, as pointed out by Wisniewska (1932), sometimes appears to rest directly on the 
floor of the valve. The other point, which was recently discussed by Rousselle 
(1965), is that the appearance of the septalium in transverse section largely depends 
on the degree of inflation shown by the brachial valve. 
Text-figure 7 shows the septalium developed in Septaliphoria arduennensis and 
also two transverse sections through the posterior part of the brachial valve of S. 
paucicosta sp. nov. These demonstrate the way in which it is possible to have a 
septalium present and yet not to have the “ diagnostic ’’ U-shaped trough in the 
hinge plates developed in transverse section. From this it is evident that consider- 
able thought should be given to the way in which shell globosity, angle of sectioning 
and relative development can cause the same structure to show considerable varia- 
tion in appearance as seen in transverse section. Transverse sections of S. pauci- 
costa rather than S. avduennensis have been illustrated as the fine detail was better 
preserved in that species. Rousselle (1965) suggested describing the septalium as 
either “‘ apparent” or “‘ non apparent ’’ depending on whether or not it appeared 
“trough shaped’”’ in transverse section. This suggestion does not seem to be 
particularly useful as a septalium is either present or it is not and its appearance in 
transverse section is obviously dependent on the factors mentioned above. 
The transverse sections of S. paucicosta clearly show the way in which the septalial 
plates are united with the median septum. This latter point has been the subject of 
some discussion. The figured photomicrograph (pl. 12, fig. 6) of an acetate peel 
proves conclusively as has been pointed out by several authors (Ager, 19650), that 
Leidhold (1920) was mistaken in describing the septalium as, “ arising from a 
bifurcation of the dorsal septum at its posterior end and fusion of the forked struc- 
tures with the hinge plate’’. The definition in the “ Treatise ’’, given above, is 
obviously the correct interpretation. A photomicrograph of a zeilleriid septalium 
is figured for comparison with that of S. paucicosta on pl. 12. 
In the genera described, the author has recognized four of the crural types so far 
defined namely, radulifer, calcarifer, arcuifer and falcifer. The author would agree 
with the “ Treatise ’’ definitions with the exception of one point concerning the 
radulifer type. In the material studied, the radulifer crural bases do not arise on the 
ventral side of the hinge plates but on the dorsal. This is demonstrated by photo- 
