UTRECHT MICROPALEONTOLOGICAL BUllETINS

The drepanodiform element (PI. 5, fig. 11) is laterally compressed. The
cusp is blade-like with sharp anterior and posterior edges. It is proclined,
whereas the anterior and posterior edges are evenly curved. These edges pass
into the antero-basal edge and the oral edge respectively, which are both
keeled. The antero-basal edge may be flexed sidewards. The basal sheath is
thin. The basal cavity is triangular in lateral view, the tip close to the anterior
edge. The oral outline of the basal cavity is straight, the anterior outline is
slightly concave. The aboral margin is straight or a little convex outwards.
The erect-scandodiform element (PI. 5, figs. lOa, b) is slig~tly bent
sidewards. The base opens to the inner side of the unit. There is no real
distinction between the cusp and the base. The anterior edge is straight to
slightly recurved, whereas the posterior edge is evenly recurved. This element
lacks an oral edge. The basal cavity is triangular in lateral view. The aboral
margin has a bulge-like extension to the inner side of the unit. The cross
section of the cusp is lanceolate with sharp anterior and posterior edges.
The acodiform element (PI. 5, figs. 12a, b) is erect to slightly recurved.
The anterior edge is flexed sidewards and forms a sharp, basally keeled
antero-lateral edge. The posterior edge is nearly in one plane with the
antero-lateral edge and passes into the oral edge. The oral edge is concave
anteriorly to become convex posteriorly. The lateral costa is strongly keeled
basally. The facets between the edges all are almost flat. All edges reach the
aboral margin, which is convex outwards between them. The basal cavity is
triangular in side view as well as in cross section. The oral outline of the basal
cavity is concave and its anterior outline is almost straight.
The roundyaform elements (see PI. 5, figs. 13a, b) are bilaterally symmetrical
and the antero-lateral edges are nicely keeled. The anterior side is a
curved plane. The' posterior edge passes into the oral edge, which i,skeeled.
The facets between the keeled edges are flat or little concave. The basal
cavity is triangular in side view. The aboral margin is straigh t or it may be
concave between the edges, which gives the latter a process-like appearance.
The oistodiform element (PI. 5, figs. 8, 9) is relatively very small and
laterally compressed. The cusp is wide and flat, carrying a rounded costa to
either side. The anterior and posterior edges are both almost straight or they
may be slightly-convex outwards. The posterior edge and the oral edge meet
at about 30°. The oral edge is convex and keeled. The base is about as high
as the width of the cusp. The aboral margin is convex outwards. The
antero-basal and postero-basal angles are rounded. The basal cavity is shallow
and its oral outline is practically straight.
Differential diagnosis: The drepanodiform and acodiform elements resemble
the corresponding elements of Drepanoistodus inaequalis (PANDER),