18 JURASSIC RHYNCHONELLIDS 
micrographs of an acetate peel of the crural bases of Septaliphoria paucicosta (pl. 12). 
In some species with radulifer crura such as Acanthothiris spinosa, the crural bases 
are only poorly differentiated. 
Calcarifer type crura have been found in the genera Thurmannella and Ivanoviella; 
the form of the crura in these genera was strictly comparable to those observed in a 
specimen of Rhynchonelloidella smithi sectioned for comparison. Although, as 
pointed out in the “ Treatise ’’, distally the calcarifer are similar in form to the 
falcifer, in mode of occurrence and proximal appearance they differ markedly. 
Text-figures showing the form of falcifer crura are given with the specific descriptions 
of Lacunosella arolica, L. sparsicosta, L. cracoviensis and L. vaga. The most striking 
difference is that, by using acetate peels, it can be seen that the early formed parts of 
the crura lie within the hinge plates of the species with calcarifer crura while in 
Lacunosella the hinge plates are never united and the early formed parts of the crura 
are external to them and never surrounded by hingeplate material (pl. 12). This 
feature also differentiates falcifer crura from the radulifer and arcuifer types, apart 
from considerations of gross morphology. The crural bases are formed by the 
deflection of hinge plate material around these early formed portions of the crura 
(pl. 12). Whether or not the crural bases are clearly differentiated depends partly 
on whether the early crus was rounded or sharply pointed, and also on how closely 
the hinge plate material followed the original shape. Although the early crus may 
have been sharply pointed, this shape may be lost completely or reduced to a 
rounded swelling on the dorsal side of the hinge plates where the latter are much 
thickened. 
Arcuifer crura are only recorded in the genus Monticlarella. Three species of that 
genus were investigated, using acetate peels, but the preservation was poor in all 
cases and no information additional to that given in the “ Treatise ’’ was obtained. 
Cardinal processes of the type found in certain Palaeozoic rhynchonellids such as 
the Uncinulidae are not developed in the material studied and, within the Upper 
Jurassic, only the genus Acanthorhynchia shows any comparable structure. In that 
genus the inner hinge plates are characteristically thickened and in some species 
this gives rise to a low rounded process on the ventral side of the hinge plates. A 
photomicrograph of the thickened hinge plates of Acanthorhynchia (Acanthorhynchia) 
panacanathina is figured on plate 12. 
Order RHYNCHONELLIDA Kuhn, 1949 
Superfamily RHYNCHONELLACEA Gray, 1848 
Family DIMERELLIDAE Buckman, 1918 
Subfamily MONTICLARELLINAE nov. 
DiacGnosis. Small rhynchonellids ; ornament includes radial striae ; ribs 
absent or variably developed ; beak small and pointed ; slightly sulcate, recti- 
marginate or weakly uniplicate ; crura arcuifer where known. 
STRATIGRAPHICAL RANGE. Lower Jurassic—basal Cenomanian. 
