UTRECHT MICROPALEONTOLOGICAL BUllETINS

Oistodus papiliosus n. sp.
(PI. 1, figs. 18-20)
Derivatio nominis: papiliosus, Lat. = like a butterfly. Referring to the shape
of the triangulariform element, seen in side view.
Holotype: The triangulariform element, figured in PI. 1, figs. 19a, b, cat. nr.
T 196 -13.
Figured paratypoids: The oistodiform element (PI. 1, figs. 18a, 18b) and the
deltaform element (PI. 1, figs. 20a, 20b). Cat. nrs. T 196 -14 and 15.
Type-stratum: "Blommiga Bladet" or "the Flowery Sheet" at about half
height the Bruddesta Formation in section 6; see chart III, sample 27.
Diagnosis: A relatively very small species of Oistodus with a butterfly-like
appearance of the triangulariform elements.
Description: All our elements contain "white matter" and are completely
white.
The oistodiform elements (see PI. 1, figs. 18a, b) are bent sidewards and
have a concave inner and a convex outer side. The base is about as long as
the cusp. The basal cavity is shallow, laterally compressed and its oral outline
is convex orally. The aboral margin is almost straight. The oral edge is keeled
and convex outwards. The antero- and basal angles both are sub-rounded.
The cusp is reclined with a sharp anterior edge which is convex outwards.
The posterior edge is keeled and has a faint sinuous curvature. A prominent
costa runs along the anterior part of the outer side of the cusp. This costa
does not reach the aboral margin.
The triangulariform element (PI. 1, figs. 19a, b) has a base with the shape
of a butterfly's wing. It is strongly compressed laterally and the basal sheath
is very thin. The basal cavity is only a groove. The aboral margin is concave
outwards. The oral edge is highly keeled and straight. The angle between the
oral edge and the posterior edge of the cusp is rounded. The entire unit is
slightly bent to one side. The cusp is reclined to recurved. The anterior edge
is sharp and bent to the inner side of the unit. The outer side of the cusp
carries a prominent costa. The latter reaches the aboral margin and forms a
flange-like extension there.
The deltaform elements (PI. 1, figs. 20a, b) are bilaterally symmetrical.
The base is reduced to a denticle-like extension at the posterior side of the
cusp. Between the oral and posterior edges there is a very sharp angle. The
antero-Iateral edges are strongly extended laterally; the cusp lacks an anterior
edge. Aborally the base and the lateral edges are flange-like in which the
basal cavity is present as a groove.
Differential diagnosis: The very small size of all elements of this species and
their bright white colour differentiate them from all corresponding elements
of other species.
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