the echinoid fauna from japan and adjacent regions part i

The Echinoid Fauna from Japan and Adjacent Regions 195
subspecies in the collection, represented by fragmentary, ambulacrum around the
peristome and a part of interambulacrum.
Ambulacrum near the peristomial edge made up of six ambulacral plates on left
·column and five ambulacral plates on right column, 5 mm wide at the peristomial
edge and 8 mm wide at fifth plate from the peristome; third plate on both columns
from the peristome made up of five components as usual type of the species, porepairs
of aboral primary, upper demi-plate and middle primary components form an
.arc of three, which is followed by an arc of two pore-pairs of lower demi-plate and
aboral primary components; the innermost is that of the lower demi-plate and placed
nearly in a row inside the pore-pairs of middle primary, and that of adoral primary
component is directly below that of lower demi- plate: this arrangement of porepairs
more or less differs from that of plates near the ambital region: this ambulacrum
seems to be narrower than that of the living specimen, the breadth of ambulacrum
at fifth plate from the peristome is 11 mm, and height from the first plate to fifth
from the peristome is equal in both the fossil and the living specimens, is 12 mm.
Tuberculation on ambulacrum is similar to that of the living specimens, but large
·crenulated tubercles seem to be more smaller,
Interambulacral plates near the ambitus more or less fractured; primary tubercles
distinctly crenulated, number of crenulation is 11, imperforate, with relatively small
base; outer and inner large secondary tubercles besides the primaries are as in those
of the living specimens, but the other small secondary and miliary tubercles seem
to be larger and more numerous than those of the living.
Owing to the poor preserved condition of this specimen, it is presumely referred
to the living species, Gly. crenularis A. AGASSIZ, as a subspecies, but it is more or
less different from the living in the ambulacrum and in the tuberculation on coronal
plates; to determine whether this difference signifies a mere variation of the living
species, or they have to be separated as a distinct species from the living, more and
.new materials are necessary.
Subgenus Eoglyptocidnris NISIY AMA n. subgen.
Type-species.-Glyptocidaris (Eoglyptocidaris) arctina NlSIYAMA, n. sp.
Test low and thin; ambulacral plates high, composed of five components, and
·divided into upper three components and lower two components; poriferous areas
narrow and arcs of pore-pairs nearly erect; interambulacral plates high; primary
tubercles imperforate, crenulate; secondary tubercles small and form vertical series
on each side of the primary series. Apical system unkown. Peristome rather
small, with broad and shallow gill-slits. Spines and pedicellariae unknown. Teeth
strongly keeled.
This subgenus is very closely related to the typical genus Glyptocidaris A.
AGASSIZ, 1863, in general features, but it differs from the typical genus by the
structure of ambulacral plates and by the tuberculation on the interambulacral
columns. This subgenus also resembles the genus Porosoma COTTEAU, 1856 (op. cit.),
in having single series of the ambulacral pores adapically, but is easily distinguished
from it by the structure of ambulacral plates and also by the tuberculation on both