the echinoid fauna from japan and adjacent regions part i

240 s. NISIYAMA
Islands.
Test small, stout, thick and subconical on aboral surface; tumid adorally and
concave toward the peristome. Apical system compact, madreporite in the largest
genital (2), oculars partly insert. Ambulacra narrow, straight; poriferous zones
narrow; ambulacra plates high, made up of three components of the echinoid structure;
pore-pairs in more or less vertical arcs of triplets; interporiferous areas with
two series of small, plain, and imperforate primary tubercles on aboral surface, with
or without irregularly placed secondary tubercles, more or less bare median area in
the ambulacra aborally; below the ambital region there is a vertical series of secondary
tubercles inside the primary series. Interambulacra broad, interambulacral
plates rather high; small, plain, and imperforate primary tubercles form vertical
series on either side of median line on aboral surface, and a distinct naked median
area in the interambulacra aborally; below the ambital region, one or two vertical
series of secondary tubercles outside the primary series. Peristome rather small ;
gill-slits distinct, sharp and deep.
This genus has characters intermediate between Tripneustes and Echinus LIN­
NAEUS, 1758, that the tuberculation on both aboral and adoral surfaces, particularly
in the naked area, resembles that of the living Tripnellstes gratifla (LINNAEUS, 1758)
(MORTENSEN, 1943, p. 500, pL 33, figs. 1-3, pI. 34, figs. 2-6, pI. 35, figs. 3-4, pI. 37, figs_
1-2, 4-10, pI. 38, figs. 1-4, pI. 56, fig. 11, text-figs. 306-307), on the other hand, the
ambulacral structure of the typically echinoid type of trigeminate plates quite agrees
with that of the species of Echinus and other allied genera. Gill-slits of this genus
are sharper and deeper than those of Echinus, thus it is easily distinguished from that
genus; but they are, in this genus, shallower and broader than those of Tripneustes_
There are some doubts that this genus might represent a young form, but the
tuberculation on both ambulacral and interambulacral areas prevents the identification
of this genus with that one.
This genus also resembles the extinct genus Gagaria DUNCAN, 1889 (DUNCAN,
1889-a, p. 91 (subgenus ad Micropsis COTTEAU, 1856).-Haplotype-Micropsis venustula
DUNCAN and SLADEN, 1884, p. 119, pI. 22, figs. 1-7-the Eocene (Khirtar Series) of
Sind), in the characters of apical system and the structure of ambulacral plates, but
is easily distinguished from that genus by having non-crenulate primary tubercles
and deeper and broader gill-slits, while in that genus the primary tubercles are
crenulate and the gill-slits shallow and narrow.
There is a single species, Mil'. mirabilis, in our collection, which described herein_
Mirechinus mirabilis NISIY AMA, n. sp.
(PI. 6, fig. 3, pI. 7, figs. 2, 4)
Holotype.-IGPS colI. cat. no. 73725.
Locality and geological horizon.-South cliff of Nishi-ura, Haha-jima in Bonin
Islands. Eorupertia-Alveolina-Zone, Upper Eocene (Lutetian).
There is a single specimen (holotype) of this species at hand. Dimensions of
holotype.-About 26 mm in horizontal diameter, and about 20 mm in height; peristome
about 8 mm in diameter.