the echinoid fauna from japan and adjacent regions part i

248 S. N ISIYA lvI A
is similar to that of Ps. depressus ; but the pore-pairs fewer in number at the adperistomial
region than in that species. Test is thick, about 2 mm thick in section ;
the larger specimen may attain to 65 mm in horizontal diameter.
Distinction.- This new species closely resembles Recent Ps. depressus (A. AGASSIZ,
1863) in the structure of ambulacra, and in the tuberculation on coronal plates; and
hence, the writer once regarded this as a variet y of that species. This species,
however, is distinguished from that species by the narrower poriferous zones, and
fewer pore-pairs in an ambulacral plate. Moreover, the arcs of pore-pairs are more
a
Fig. 22. Pseduocenl rotus del;ressus (A. AG ASS IZ, 1863) (for comparison)
. (repS co li. cat. no. 78228- 1, from the sou t h coast of Tateya
m a Bay, Chiba Prefecture) . x 6.
a. A n ambul acrum a t the mid zo ne ; b. A n a mbulacrum on t he pe ristomiai
regio n.