FROM NORTHWESTERN EUROPE 15 
In order to help clarify the distribution patterns, text-fig. 2 shows the outcrop of 
the Upper Jurassic (including the Callovian) and also indicates a generalised palaeo- 
geography of Oxfordian-Kimmeridgian times, after Kuhn (1953). Volgian times 
saw a major withdrawal of the sea from much of the area and the consequent marked 
decrease in the rhynchonellid faunas can be seen by comparing text-figs. 4 and 5. 
This decrease is particularly evident when it is considered that most of the German 
fauna plotted on text-fig. 5 is probably restricted to the Gravesia and Subplanites 
zones of the basal Volgian. 
The stratigraphical ranges of the Oxfordian-Volgian species studied are tabulated 
on text-fig. 6. These ranges have been determined as accurately as possible, but it 
has often proved difficult to translate records from older stratigraphical papers into 
modern zonal terms. 
MORPHOLOGY 
The present author has accepted the definitions of morphological terms used in 
the “ Treatise ’”’ (1965) with the exception of the following points : 
Within the species studied, it has been found when describing the form of the 
pedicle opening the choice of term has invariably been limited to either hypo- 
thyridid or submesothyridid. In many cases it is extremely difficult to differentiate 
these types ; in none of the species examined did the pedicle opening strongly 
impinge on the beak ridges. The second point concerns the pedicle collar ; accord- 
ing to the definition in the “ Treatise’, this is an internal feature of the ventral 
beak, “continuous laterally with the internal surface of the deltidial plates ’’. 
However, the feature figured by Thomson (1927) under that name consisted of a 
groove running from the posterior side of the pedicle opening to the tip of the beak. 
This latter structure has been found in the genus Acanthothiris and the term 
“pedicle trough ”’ is proposed for it. 
In describing the internal structures, care has been taken to stress when the 
features are being described as seen in transverse section and also to avoid mis- 
leading descriptions such as “ septum long ”’ when in fact it is high. The difficulties 
present in interpreting transverse sections are further discussed below with reference 
to the septalium. 
The septalium has recently been defined in the “ Treatise ’’ as being a “ Trough- 
like structure of the brachial valve between the hinge plates (or homologues), con- 
sisting of septalial plates (or homologues), enveloping and buttressed by median 
septum ; does not carry adductor muscles’’; the author would agree with this 
definition. However, it has been thought necessary to discuss this structure in 
some detail for two main reasons ; firstly, the author has collected silicified material 
of Septaliphoria arduennensis and Thurmannella obtrita, the species in which this 
structure was originally described by Leidhold (1920), and secondly, because there 
seems to be some confusion in the literature as to the appearance of the septalium as 
seen in transverse section. 
All the elements which together constitute the septalium are capable of consider- 
able variation. This point would seem to be self-evident and yet has undoubtedly 
