File:The Quarterly journal of the Geological Society of London (14076831084).jpg

54
PEOr. E. W. CLATPOLE ON PTEPvASPIDIAN FISH IX
above them in the upper division of the vascular layer, and below
the minute channels of the upper and outer surface, with which
they are apparently continuous. These highest canals, to which
possibly the term furrows would be more applicable, have no
branches or outlets, so far as I have been able to determine, except
the passages just mentioned, nearly as wide as themselves, whereby
communication is effected with the vessels lying below them, and
already described. Both may be readily seen sectionaUy and longi-
tudinally by cutting thin slices of the shield in the proper direction
(see figs. 2, 3, 4).
■Sections of the Shield of Palseaspis.
Kg. 2.
I^ig. 3.
Fig. 2. Vertical longitudinal section showing (a) basal layer ; .b) cellular layer ;
(c) vascular layer, ..) lower portion, (^) upper or tubular portion.
Fig. 3. Vertical transverse section, showing (a) basal layer ; (6) cellular layer ;
(c) vascular layer, Q-) lower, (^) upper portion.
Fig. 4. Horizontal section through vascular layer of shield, showing the lower
branching vessels, the upper vessels or channels, and the tubules (seen
from below).
Lastly, there yet remains to be described the most remarkable
part of this outer or upper vascular layer. To this I have given
the name of the tubular division. From the larger and lower of the
two sets of vessels just described and from its upper side there issue
a vast number of minute tubules which branch and run to right and
left, and upward from the parent vessel, forming a minute and beau-
tiful forest-like growth. Pilled as they are with opaque matter
they stand out distinctly, like dendrites on marble, on the light yel-
low surface of the material of the vascular layer. They do not form