79 



of the ambulacra, outside the pores, is reddish. The spines are green, often red- 

 dish brown in the middle, whitish in the outer part; they are not ringed. 



Off Tung Kaben, 6 fathoms; sandy mud (6 specimens). Koh Mesan, 5 — 9 

 fathoms (10 specimens). A specimen was taken at Singapore by Mr. Gad. Prof. 

 DÖDERLEIN kindly tells me he has a specimen of this species from Samoa. 



This beautiful and very interesting species is very distinct from all the 

 other species of Pleurechinus (comp, those described below). It is above all distin- 

 guished by the very remarkable feature of having unpaired buccal plates and 

 buccal tube feet. (Also in young specimens of Hypsiechinus and Prionechinus may 

 be found only 5 buccal tube feet, but there are always 10 buccal plates. I have 

 also seen a specimen of PL bothryoides in which one of the buccal plates, in one 

 pair only, was rudimentary and without tubefoot, the other plate bearing two feet; 

 the other pairs were normally developed. Likewise in a specimen of PI. variegatus 

 1 have found the one tubefeet wanting in one of the pairs of buccal plates.) Possibly 

 it ought to be made the type of a separate genus; but as it is otherwise a typical 

 Pleurechinus, I think it best to retain it in this genus. If in the future more species 

 with this disposition of the buccal plates and tube feel should be made known, 

 there might be more reason to make a separate genus for their reception. 



13. Pleurechinus siamensis n. sp. 



PI I. Figs. 2, 7, 11, 20. PI. II. Figs. 2, il, 14, IT), 22. PI. VI. Figs. 1(>, 36. PI. VII. Figs. 14, 44, 53. 



The test is low, 5 mm. high in a specimen of 7-5 mm. in diameter. The 

 actinal side is flat, not distinctly incurved at the peristome; mouthslils (juite indistinct. 

 Peristome 32 mm. to a diameter of test of 7-5 mm., c. 40°/o of the diameter of test. 



The ambulacra (PI. II. Fig. 2) are '^k as broad as the interambulacra. The 

 pores are very small, disposed in a single straight series and close to the edge of the 

 area. The poriferous zone is very narrow, without any tubercles, scarcely Va so 

 broad as the interporiferous zone. The ambulacral plates are as high as the inter- 

 ambulacral ones, their number being the same, 9 — 10 in a specimen of 7-5 mm. 

 diameter. The primary tubercles, one to each compound plate, form a distinct 

 series close to the poriferous zone; they are of the same size as the interambulacral 

 primary tubercles. The base of the primary tubercles of both areas is a little stellate 

 reminding one of Genocidaris maculata. (Also in PI. Döderleini there may be a very 

 faint indication of a stellate condition.) Secondary tubercles rather few and small, 

 only at the ambitus they may be distinct, forming a small inner series on a few 

 (3 — 4) plates. The pits are large, deep, rectangular, with the ends a little rounded. 

 Also on the lower side they are distinct, except on the first 1 — 2 plates. The pores 

 along the edge of the ambulacra very indistinct. 



In the interambulacral areas (PI. II. Fig. 9) the primary tubercles are, as in 

 the ambulacra, a little stellate, disposed in a distinct longitudinal series. The 



