the echinoid fauna from japan and adjacent regions part i

158 S. NISIYAMA
general conspicuously sinuate; pores non-conjugate, the wall usually forming a low,
rounded crest. Madreporite not distinctly enlarged. Pores on peristome in a single,
regular series. Primary spines often conspicuously flaring at the tip; the shaft
usually with a spongy coat of hairs, more rarely with simple, unbranched hairs;
collar short, neck very conspicuous, about twice as long as the collar. Scrobicular
spines form a close mail round the base of primaries; miliary spines scale-like, appressed,
or spiniform, erect. Large globiferous pedicellariae without end-tooth and without
a limb on the stalk; small globiferous pedicellariae usually without end-tooth,
but sometimes with a distinct, though small end-tooth; they may be very elongate
and slender, resembling tridentate pedicellariae, and in this case true tridentate pedicellariae
are lacking. Spicules of tube-feet may be transformed into larger fenestrated
plates; spicules of intestinal wall numerous, small, triradiate, or reduced to very
small rods, simple or slightly branched, in which case they are exceedingly scarce.
Moderate to usually large forms, ranging from the Cenomanian to the Recent
seas (from MORTENSEN, 1928-b).
This is the poorly defined and perplexing genus in the family, usually recognized
by the absence of primaries on the uppermost coronal plates, but this is not marked
in small specimens. The resemblance to Cidaris LESKE, 1778, on the one hand, and
to Austrocidaris H. L. CLARK, 1907, and to Goniocidaris AGASSIZ and DESOR, 1846, on
the other hand, is great. Often the primary spines carry ridges or 'rings' on the
basal part, and usually they are flaring at the tip, but they may be cylindrical or
terete. The apical system is large, often convex, and noticeably thick and stout, and
usually thickly and uniformly covered with tubercles of approximately equal size.
This and the primary spines are the best characteristic for separating the genus
from Austrocidaris, besides the character of the pedicellariae. Recent forms of Stereocidaris,
essentially an Indo-Pacific, ranging from South Africa to Japan and eastwards
to the Hawaiian Islands. But one isolated species (ingolfiana MORTENSEN) occurs in
the North Atlantic from Iceland to Nervis in 165-665 fathoms. As to the distribution
and history of species of this genus, MORTENSEN (1928-b, p. 230) suggested a possibility
that the genus originated in the European seas of the Cretaceous and, when
it was about to vanish there, found its way to the Indo-Pacific to reach there the
flourishing condition of Recent days. This is a very interesting suggestion and may
be near the truth, but a number of fossil species (about 20) from the Cretaceous
System (from the Cenomanian to Senonian) are referred to this genus, and the interrelationships
of them to the Recent species are somewhat dubious, and moreover,
there is a trace of occurrence of species in the Cretaceous system of Japan. All the
Recent species usually occur in depth of 70 metres or over.
The existence of more or less distinct grooves at the median end of the horizontal
interambulacral sutures is very interesting, and suggests Goniocidaris affinities of
Stereocidaris. Also the structure of the pedicellariae and the character of the spicules
in the intestinal wall suggest affinities with the Goniocidaris. The suggestion, therefore,
is at hand that the Goniocidarids may have been derived from Stereocidarid
ancestors. The fact that Goniocidaris is a very Recent type, not known with certainty
as fossil, is in good accordance with such suggestion.
Three species and three subspecies of the living Stereocidaris are reported from