the echinoid fauna from japan and adjacent regions part i

196 s. NISIYAMA
ambulacra and interambulacra.
Glyptocidaris (Eoglyptocidaris) arctina NISIYAMA, n. sp.
(PI. 3, figs. 5-7, text-fig. 7)
Holotype.-IGPS colI. cat. no. 73752.
Locality and geological horizon.-IGPS loco no. Te-24.-A point of Sukenebetsu,.
on the upper course of the River Obirashibe, Obirashibe-mura, Rumoe-gun, Teshio
Province, Hokkaido. (tm Neiraku, ca, 44°03'N., Long. 141°55'E.). Poronai formation,
Oligocene (or Eocene).
There is a single specimen referred to this new species at the writer's disposal,
but the aboral surface is damaged; the test measures about 60 mm in horizontal
diameter and about 30 mm in height.
Test low and coronal plates very thin, less than 0.5 mm thick at the ambitus.
Ambulacra narrow, about 14 mm wide at the ambitus and narrowed gradually
towards the peristomial edge, where breadth is 5 mm; ambulacral plates high, about
7 mm wide and 6 mm high at the ambital region, 10 or 11 plates on each ambulacral
column from the ambitus to the peristomial edge; compound plates of peculiar mode
of five components; these five components are divided into upper (aboral) three and
lower (adoral) two; upper three are composed of aboral primary, upper demi-plate
and middle primary components (as in the echinoid structure) ; lower two composed
of lower demi-plate and adoral primary components (text-fig. 7). Poriferous zones
narrow, about 2 mm wide at the ambitus, arcs of pore-pairs not so oblique but nearly
erect near the ambital region; upper three pore-pairs of aboral primary, upper demiplate
and middle primary components form a nearly vertical arc, the slightly outermost
is that of the upper demi-plate and the slightly innermost is that of the
middle components; in arc of lower two components, pore-pair of lower demi-plate
directly below that of middle primary component and pore-pair of adoral primary
component further slightly inside than that of lower demi-plate; pore-pairs become
a single series adapaically. Primary tubercles on ambulacrum large, distinctly
crenulate, imperforate, number of crenulation is 10, placed somewhat aborally on
plates and cover sutures of aboral primary and of upper demi-plate components,.
but not of middle primary component, thus lower (adoral) two components are free
from the large primary tubercles even in the ambital region; small secondary
and miliary tubercles placed inside the pore-pairs and irregularly around the primary
tubercles; primary tubercles form vertical series on each side of ambulacral median
Fig. 7. Ambulacral structure
of Eoglyptocidaris aretina
NISIYAMA. x3
line, and continue to the peristomial region, and gradually
decrease in size; lower (adoral) two components
of ambulacral plates on oral surface are always distinct
from upper (aboral) three components, and with large
secondary tubercles below, which are placed somewhat
outside the primary tubercles.
Interambulacra broad, about 22 mm wide at the
ambitus; interambulacral plates rather high, about
11 mm wide and 7 mm high at the ambitus, 8 or 9 plates.