77 



12. Pleurechinus Döderleini n. sp. 



PI. I, Figs. 12. 13. PI. II. Kigs. 1. 7, 8. PI. VI. Figs. 35, 43. PI. VII. Figs. lU, 48. 



The form of the test is rather variable. The hirgest specimens (12 mm. in 

 diameter) are rather high (10 mm.) and conical, the smaller ones are generally 

 more flat (7 mm. high to lU mm. in diameter for instance). The lower side is rather 

 flat and a little incurved at the peristome. 



The ambulacra (PI. II. Fig. 8) are only a little narrower than the interambu- 

 lacra. The pores are rather large, disposed in an almost straight series, the middle 

 pair in each compound plate being placed scarcely so much as half the width of 

 a single pore outside the two other pairs; they are not placed close to the edge of 

 the area, in the largest specimens there are even some small tubercles outside the 

 pores. The interporiferous zone is twice as broad as the poriferous. The primary 

 tubercles — one to each compound plate — form a distinct series close to the pori- 

 ferous zone; above each primary tubercle there is a smaller secondary one, placed 

 in the same line as the primary ones. On the inner part of the plate there may 

 be (in the larger specimens) a pair of secondary tubercles, or this part of the plate 

 may be covered by miliary tubercles. The ambulacral plates are as high as the 

 interambulacral ones, the number of plates being almost exactly the same in both 

 areas (12 — 13 in specimens of 10~ 12 mm.). The pits are large and deep, rectangular, 

 reaching to the base of the primary tubercles; they are indistinct on the 2—3 

 lowermost plates. The pits along the outer edge of the ambulacral plates are di- 

 stinct, those in the sutures between the compound plates being distinctly larger 

 than those at the sutures of the primary components of the ambulacral plates; they 

 sometimes unite with the outer interambulacral pits, which are of the same size 

 as the ambulacral ones. 



In the interambulacral areas (PI. II. Fig. 7) the primary tubercles likewise 

 form a very distinct series. The secondary tubercles are not arranged in a hori- 

 zontal row on each plate; there may be 2—3 larger ones outside and inside the 

 primary tubercle and one above the primary tubercle at the upper side of the 

 plate. The secondary tubercles of each plate thus form an oblique series on 

 each side of the primary tubercle, meeting at a right angle in the tubercle 

 placed above the primary one. The miliary tubercles may be rather scarce or they 

 may occur in great numbers, covering the whole plate; in the latter case the 

 secondary tubercles are very inconspicuous, and hardly larger than the miliary 

 ones. The pits are very large and deep, leaving only a narrow band free in the 

 middle of the horizontal suture, as broad only as the base of the primary tubercle. 

 Both the inner and outer pits are a little widened towards this pari. A distinct 

 covering membrane may be seen along the edge of the pits (comp. PL siamensis). 



The apical area (PI. II. Fig. 1) is small, 25 mm. in diameter in a specimen 

 of 10 mm. diameter of test. At the inner end of the ocular plates there is a distinct 



