FROM NORTHWESTERN EUROPE 49 
DIsTRIBUTION. The species occurs quite widely in the lower Oxfordian of the 
northern rim of the Paris Basin, in the northern part of the French Jura and in the 
Swiss Jura around Basel. T. obtrita seems to be co-extensive with the “ Terrain a 
Chailles ”’; whether it is equally widespread within the Renggeri Marls is not known, 
although the remarks of Loriol (1900) would suggest that it is. 
OccURRENCE. The species has been collected in quantity from two horizons 
represented by three markedly differing lithologies. The lower horizon is represented 
by the Renggert Marl, which is a dark coloured calcareous clay. The specimens 
collected showed a very wide range of size and it is presumed that all except the very 
youngest stages are present. It seems reasonable to assume that the Marl was 
deposited in a fairly low energy environment. 
The Renggeri Marl belongs to the mariae zone, while the higher horizons at which 
T. obtrita occurs are in the cordatum and transversarium (lower part) zones. The 
two distinctive lithologies occurring in the upper zones are the stratigraphically 
equivalent ‘‘ Terrain a Chailles’’’, a sandy limestone with nodular layers of silica, 
and the self explanatory “ oolithe ferrugineuse ”’. 
The specimens of 7. obtvita found in the “ Terrain a Chailles ”’ tend to be con- 
siderably larger than those from the Marls, both in average size and in the absolute 
size of the largest specimens ; it is also noticeable that there are fewer of the very 
small individuals. It is suggested that the average size of the specimens within the 
species probably increased through its stratigraphical range and that the larger size 
of the specimens from the “ Terrain a Chailles ’’ is emphasized by the higher energy 
environments which is assumed from the lithology ; the smaller shells were probably 
carried away. 
The “ Terrain a Chailles ” is coarsely bedded and the tops of the bedding planes 
are rather nodular and often show evidence of burrowing activity. T. obtrita is 
found predominantly on these nodular surfaces, where it occurs both singly and in 
groups. Nearly all the specimens are present with complete, articulated shells, 
suggesting that they were not transported far before burial ; this assumption is 
supported by the very delicate nature of the test. 
From the thin shell and the large, obviously functional pedicle opening, it is 
suggested that the individuals may possibly have lived attached to weed and that 
on death they became detached and dropped to the bottom. The groups could be 
either post-mortem accumulations washed together or possibly small colonies which 
were deposited together from the same fronds of weed ; if they in fact adopted that 
mode of life. The “ Terrain a Chailles”’ deposits are thought to have accumulated 
in water no deeper than about 10 fathoms and possibly quite close to the shore line. 
The above remarks on the “ Terrain 4 Chailles ”’ are probably also largely applic- 
able to the “ oolithe ferrugineuse ”’, which is about 7 m thick in the area of Verdun 
where the specimens were collected. The bed is only poorly exposed, largely in old 
shell craters, and collecting from it was relatively difficult. T. obtvita does not seem 
to be so abundant as in the “ Terrain a Chailles ’, but the specimens are about the 
same size. Associated fauna included terebratulids, irregular echinoids (?Collyrites), 
and lengths of Millerecrinus. 
