the echinoid fauna from japan and adjacent regions part i

254 s. NISIY A:\IA
Locality and geological horizon.-IGPS loco no. Sr-4.-Akatamobuchi on the River
Komebuto, about 2 km south of Kuromatsunai railway station, Kuromatsunai-mura.
Suttsu-gun, Shiribeshi, Hokkaido. (tm Oshamanbe, Lat. 42°38'56/1N., Long. 140°19'53/1
E.). Set ana formation, Pliocene. Hypotype.-'-IGPS colI. cat. no. 73796.
Locality and geological horizon.-IGPS loco no. Sr-12.-A point of Miyakawa,.
Kami-Iso-mura, Suttsu-gun, Shiribeshi, Hokkaido (exact locality unknown to the
writer). (tm Oshamanbe, ca. Lat. 42°36'N., Long. 140 0 27'E.). Setana formation, Pliocene.
Hypotype.-IGPS colI. cat. no. 73814.
There are a few fragmentary specimens referred to this species at disposal;
they can be assigned to Str. echinoides on account of the characteristics of the
pore-pairs and the tuberculation on the plates.
Ambulacra rather narrow, about 15 mm wide near the ambital region; ambulacral
plates rather high, about 7.5 mm wide and 2.7 mm high near the ambitus; composed
of seven components, e. g., aboral primary, upper aboral demiplate, lower aboral
demi-plate, middle demi-plate, upper adoral demi-plate, lower adoral demi-plate, and
adoral primary; poriferous zones rather narrow, being nearly half of the interporiferous
areas; three pore-pairs of aboral primary, upper aboral demi-plate, and
lower aboral demi-plate with a pore-pair of adoral primary of successive upper (aboraD
plate form a nearly horizontal, or somewhat inclined arc; four pore-pairs of lower
aboral demi-plate, middle demi-plate, upper adoral demi-plate, and lower adoral demiplate
form an arc, which is nearly vertical, or strongly inclined. Primary tubercles
placed centrally and slightly adorally, and close to the poriferous zone; they form
rather conspicuous vertical series on each side of median line; large secondary
tubercles form a vertical series inside, halfway between median line and the primary
series; smaller secondary tubercles also form two vertical series, one close to median
line and the other to middle part of the poriferous zone near the ambital region;
small tubercles and miliaries on the poriferous zone numbering about 12, and those
on the interporiferous area over 30, and the total over forty on an ambulacral plate
near the ambital region.
Interambulacra rather broad, about 20 mm wide near the ambital region; interambulacral
plates higher than the ambulacral plates, about 10 mm wide and 3.8 mm
high near the ambital region; primary tubercles placed somewhat centrally and
adorally, and form a rather conspicuous vertical series on the middle of a column;
large secondary tubercles, which are much smaller than the primaries, form two
vertical series one inside and the other outside the primary series; of these two,
the inner series composed of two rows, one placed adorally and the other aborally
on a plate near the ambital region; small secondary tubercles and miliaries rather
numerous, over 45 of them being outside the primary tubercle, and even more than
50, inside: nearly 100 in total.
This species so closely resembles Str. droebachiensis (0. F. MULLER, 1776) (loc.
cit.) that some writers consider that the former is only a form or variety of the
latter. The former, however, may be distinguished from the latter by the higher
ambulacral plates, the more inclined arcs of pore-pairs, which number seven against
six, and by the more numerous tubercles on the plates.