Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...

38 ADVANCES IN PALAEOICHTHYOLOGY
visible at the broken edge. It seems probable that this embayment is the same as that
seen on the postero-lateral part of the dorsal disc in Obruchevia, possibly indicating
the position of the branchial duct.
CMN-NUFV105 (Fig. 8A, 8B) is almost flat and oval with one straight broken
margin and with maximum dimensions of 140 mm x 207 mm. The two longest margins
curve slightly, so that one is slightly concave, the other slightly convex. The thickness
varies from 15 mm to 10 mm. The external surface (Fig. 8B) is slightly convex, and
though heavily weathered, an ornament of small blisters can be seen in some places. No
sensory canals are visible. The inner face (Fig. 8A) is smooth and shiny and a number
of smooth, rounded blisters are present, generally elongated in a radial direction. A
series of growth lines is visible. These seem to parallel the convex margin but are only
sub-parallel to the concave margin, dying out successively against it. This specimen is
interpreted as the left posterior part of the ventral plate. The concave margin forms one
side of the median posterior notch.
The histology can be seen in thin section (CMN-NUFV106, Fig. 9) and in the weathered
broken edges of some of the plates. The lamellar basal layer, up to 2 mm thick, is
composed of thin horizontal lamellae through which pass roughly vertical vascular
canals (Fig. 9C, lam). This is similar to the situation in Obruchevia, although it appears
to be a thinner and less compact layer (Obruchev 1941, pl. 2-1). The main part of the
plate consists of a spongy layer composed of aspidin, with openings varying in size
from 0.5 mm near the base to 2-2.5 mm in the upper part. The outer 2.5 mm of the plate
Fig. 8. Perscheia pulla gen. et sp. nov. Left posterior part of ventral plate (CMN-NUFV105) in
dorsal (A) and ventral (B) view. Scale bar equals 50 mm.