Bd. VI: 4) THE ECHINOIDEA. 63 



and a few primary tubercles. — a conspicuous difference from the smooth plates in 

 the two other species. 



The primary spines are rather coarse, pointed, slightly curved, with none or 

 only a few serrations; the spines of the actinal plastron are likewise pointed, not 

 widened in the end; those around the peristome are not flattened. They all stand 

 out very prominently, white against the more or less violet ground colour of the 

 test. Those of the subanal plastron are the longest, 2.5 mm. in a specimen of 9 mm. 

 length. — The secondary spines are widened at the point and somewhat curved 

 (PI. XVIII Fig. 9). 



The tubefeet on the inner circle of plates round the peristome are penicillate, 

 with only 1 — 4 thick filaments, supported by very strong, irregular, fenestrate rods 

 (PI. XVIII Fig. 10). No spicules were found in these tubefeet. The simple tubefeet 

 following next have a small cap of calcareous network in the point, and some few 

 simple spicules occur in their walls. 



The pedicellariae are rather scarce; only tridentate, ophicephalous and triphyllous 

 pedicellariae have been observed. They are upon the whole very similar to those 

 of PL hirsutus, the tridentate ones being only a little more irregular (PI. XVIII 

 Figs. 6, 11); bivalve specimens occur. The triphyllous pedicellariae (PI. XVIII Fig. 7) 

 are like small tridentate. A form corresponding to the globiferous pedicellariae of 

 PL hirsuttis I have not found. The ophicephalous pedicellariae (PI. XVIII Figs. 5, 12) 

 are more developed than those of PL hirsutus, the blade being somewhat broader; 

 the upper end of the stalk cup-shaped as usual. 



Sphaeridiae occur only on the plates nearest the peristome, also in the anterior 

 ambulacrum. They are short, round, with rather prominent ridges (PI. XVIII Fig. 8). 



The colour was brownish-violet in the one specimen, white with faint traces of 

 violet pigment in the other. 



Two specimens were taken at Station 17, on the Shag Rock Bank (53° 34' S. 

 43 0 23' W. 160 m. — Sand, gravel; bottom temperature + 2.05). The measure- 

 ments are: 



Length. Breadth. Height. 



12 mm. 9 mm. 7 mm. 



9 » 6.5 » 6 . 



It may be especially noticed that also the smaller specimen has its genital papillae 

 developed. It is thus probably a very small form. 



This very interesting species differs conspicuously from the two other species 

 known of the genus Plexechinus, PL ductus and hirsutus, especially by its primitive 

 apical system, by the fasciole comprising only 3 pairs of interambulacral plates and 

 upon the whole by the structure of its epiproctal region. Perhaps these characters 



