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30 ADVANCES IN PALAEOICHTHYOLOGY
Specimen Pl 10/10 is a fragment of Obruchevia ventral plate (Fig. 4C, 4D) from the
posterior portion of the right side; it is 110 mm wide, 105 mm long, and has a maximum
thickness of 17 mm. The margins of the fragment are broken, except for the smooth
and slightly convex mesial margin that shows the presence of a posterior notch. Five
growth lines are visible on the external surface, the middle one being the strongest.
They parallel the outer margin of the plate but successively die out against the inner
margin. The two earliest growth zones are smooth but the later ones have short radial
Fig. 3. Obruchevia heckeri. Fragments of the dorsal plate showing variations in dorsal surface
ornament and the presence of growth lines. 1, PI 10/11; 2. PI 10/12. Scale bars equal 50 mm.
furrows. The fragment is thickest (about 20 mm) at its anterior margin and above the
earliest two growth lines. The plate becomes gradually thinner towards the lateral
margin. The ventral plate was probably rather flat. As in Obruchevia dorsal plates,
the growth of the ventral plate took place along the outer edges under the margins in
such a way that one or several flanges (actually growth zones) were formed. The
center of the ventral plate and the flanges that were formed later are thinner. Due to
this method of growth the spongy layer is laminated. Weathering of the plate has
caused the thin basal layer on the visceral surface of the ventral plate to peel off and
the spongy layer has also peeled off in patches (Fig. 4D). The ventral plate, not
known before in Obruchevia, had a deep posterior notch similar to that found in
Schizosteus, Pycnolepis, Pycnosteus, Ganosteus, and Tartuosteus (see Obruchev
and Mark-Kurik 1965; Halstead Tarlo 1964, 1965) (Fig. 10B).