UTRECHT MICROPALEONTOLOGICAL BUllETINS

carina or costa on the outer side IS prominent. The antero-basal angle is
strongly extended antero-aborally.
The oistodiform elements (see PI. 7, fig. 4) are strongly compressed
laterally and almost straight. The base is lower than the width of the cusp.
The basal cavity is shallow and its oral outline is slightly convex orally.
The aboral margin is practically straight in side view. The oral edge is keeled
and convex outwards. The antero- and postero-basal angles are rounded.
The cusp is wide and thin. A rounded costa runs along both lateral sides.
The anterior and posterior edges are both keeled. The anterior edge is convex
outwards, whereas the posterior one has a sickle-shaped curvature. The angle
between the oral and posterior edges varies between 20° _50° .
The triangulariform elements (see PI. 7, figs. 3a, b) are asymmetrical and
have two prominent, process-like edges, one lateral and one antero-Iateral.
The basal ends of these lateral edges protrude as undenticulated processes.
The basal cavity extends into these process-like edges and its oral outline
is straight. The oral edge is highly keeled and convex. Usually it is rather
strongly twisted outwards and sidewards. The lateral edge forms a costa up
on the cusp; basally it bends evenly in posterior-aboral direction. The anterolateral
edge is evenly convex outwards. The posterior and oral edges meet at
an angle of approximately 70°. The facet between the antero-Iateral and
lateral edges is convex outwards high up to the cusp, towards the base it
becomes flat to slightly concave. The cross-section of the cusp is triangular at
mid-height, whereas it is subcircular at the top. It may be hard to distinguish
this element from the corresponding element of Oelandodus costatus n. gen.
n. sp. However, the latter has a rounded costa to both sides of the base of
the oral keel.
Regional occurrence: At Kopingsklint the species was not found. In section
6 it occurs from the base of member Ka-d 1 up to and including the basal
portion of member KkJ. Near Horns Udde from the topmost portion of
member Kk2 up to halfway member Kk3'
Biostratigraphic range: Thi:s-:;pecies was found from the Prioniodus elegans/
Oelandodus elongatus Assemblage Zone up to and including the Oistodus
lanceolatus /Prioniodus deltatus Assemblage Zone (fig. 15).
Oelandodus costatus n. sp.
(PI. 7, figs. 5-7)
Derivatio nominis: costatus, Lat. = with costae.
Holotype: The elongatiform element, figured on PI. 7, figs. 5a, b, cat. nr. T
195-13.
Figured paratypoids: The triangulariform element (PI. 7, figs. 6a, 6b) and the