MILIOLIDAE - Acta Palaeontologica Polonica

MILIOLIDAE FROM MIOCENE OF POLAND 75
sides, which may be slightly depressed along the keels, ornamented with
two sharp carinae placed wide apart on the periphery of the last chambers,
the edge of the large middle chamber forms a prominent longitudinal
keel coincident with the coiling axis; sutures distinct, depressed; surface
rough, with fine irregular longitudinal grooves; aperture circular, often
at the end of a short extension of the chamber, with a short bifid tooth.
Variability. The great morfological variability is expressed in a strong
or weak development of two peripheral edges. Typical forms (Part I,
PI. XII, Fig. 3; Part II, Text-fig. 26/3, 4) are regular, having sharp keels
lmm
~~~
k~b ~5bt:Jid 6b
Fig. 26. Morphological variability and internal structure of Cycloforina contorta
(d'Orbigny), Upper Tortonian, Gliwice Stare; 1,2 - forms with bluntly keeled chamber
edges; 3, 4 - typical forms; 5, 6 - forms with irregular carinate chamber edges;
a front view, b apertural view, c cross-section.
and a characteristic axial position of the middle-chamber edge. The irregular
forms may have blunt and sinuous chamber edges (Text-fig. 26/1, 2).
Dimorphism and ontogeny. There are such small differences in size
between the proloculi of specimens investigated (20-5011), that they may
be regarded as belonging to the micro generation. The chambers of the
juvenile stage are tubular, the next chambers, from the 6th or 7th onward,
are angular in section and usually with two, exceptionally with one keel.
Remarks. - Forms with rounded edges are morphologically close to
Cycloforina gracilis (Karrer), to which they are probably related, having