FROM NORTHWESTERN EUROPE 47 
strongly developed ; is it accompanied by the development of a low fold on the 
brachial valve. However, the development of the uniplication is very variable and 
it may be low and rounded even in large specimens. The outline of adult specimens 
is subpentagonal. 
The beak is suberect in the young specimens but becomes characteristically erect 
in the adult ; it is always relatively small and sharply pointed. At all stages the 
species possesses a well developed, submesothyridid pedicle opening. The beak is 
flanked by sharp beak ridges which clearly delimit an incurved interarea. The 
deltidial plates may be either conjunct or disjunct. 
The 15-20 ribs, which are only present on the anterior half to a third of the shell, 
are low and sub-rounded to sub-angular. The ribs appear to be consistently more 
rounded on the material from the “ Terrain a Chailles’’; however, this may, at 
least in part, be a result of the different preservation. At the extreme anterior of 
the largest specimens, the ribs sometimes develop a weak trough along their crests. 
DIMENSIONS OF FIGURED SPECIMENS. 
length thickness width 
I-42 cm I-17 cm 1-53 cm 
I'73 cm I-70 cm I-93 cm 
I-40 cm 1-35 cm 172" cil 
Internal characters. The specimen sectioned was largely silicified with a conse- 
quent loss of much fine detail internally. 
Pedicle valve. The subquadrate delthyrial cavity is flanked by strong subparallel 
dental lamellae, which persist, as seen in transverse section, until the teeth have 
been completely inserted in their sockets. The teeth do not appear to be crenulated ; 
lateral denticulae are not developed. 
Brachial valve. A septalium is present, although, judging from the sectioned 
specimen and from the silicified material treated with acid, both the median septum 
and the septalial plates vary considerably in their development. However, some of 
this apparent variation is almost certainly a result of the incomplete silicification. 
Crural bases are well developed ; inner and outer socket ridges are both present. 
The crura appear to conform in general appearance with the “ calcarifer’”’ type 
described by Muir-Wood (1936). Distally the cura are blade-like and falciform, as 
can be seen in text-figure 19, however, at their posterior ends they are bent inwards, 
at almost right angles to their distal ends, to form a “ flange”’. These “ flanges ”’ 
are undoubtedly the structures Leidhold (1920) figured and described as “ Crural- 
spitze’’. This feature differentiates the crura from the true falcifer type as 
developed in Lacunosella, and it seems that it is the diagnostic feature of calcarifer 
crura as defined by Muir-Wood (1936). In her original description Muir-Wood 
described calcarifer crura as consisting “‘ of two flattened, curved, posteriorly con- 
cave laminae which project from the hinge plate into the cavity of the dorsal valve. 
These laminae each unite with a second curved lamina which appears to be suspended 
from it and projects dorsally like a spur. A ventral extension of this second lamina 
terminates in a hook-shaped process, the apex of which is directed medianly ”’. 
