CARAPACE SCALE PLATES.—Eospinus has enlarged tuberculate
scale plates over the entire body, the largest of which are
between 12% and 13% SL and tend to form a continuous
carapace around the middle of the body, although the plates are
not as regularly hexagonal or extensively sutured to one
another as in ostraciids. In ostraciids the mostly hexagonal
tuberculate scale plates are firmly sutured to one another to
form a rigid carapace over most of the body, with scale plates
of up to 13% SL (larger in the Eocene Proaracana, up to 24%
SL).
In spinacanthids one genus, Spinacanthus, has moderately
enlarged scales of 4.8% SL, while the other, Protobalistum, has
greatly enlarged scales of 12.3% SL, and in both cases the
scales bear numerous tubercles on the surface, like those of
Eospinus and the ostraciids. In Spinacanthus the plates are
more or less isolated and only cover a total of about one-fourth
of the surface of the body, without much contact between the
plates except ventrally. In Protobalistum, however, the scale
covering is much more complete, like that of Eospinus. In the
anterior half of the body (exclusive of the head) of Protobalistum
many of the plates are distinctly hexagonal and articulate
with one another by interdigitations, just as in ostraciids. This
is similar to the situation in Eospinus, in which the greatest
consolidation of the scale plates and their closest articulation to
one another is in the anterior half of the body below the spiny
dorsal fin and in front of the pelvic spine and the posteriorly
oriented carapace spine.