66 



lateral looth (PI. VI. Fig. 2). The large form, which has also been observed bj' de 

 Meliere, is almost exclusively found on the upper part of the test, where it is very 

 conspicuous; the small form is also very numerous at the top of the test, but occurs 

 more spread over the whole test. The tridentate pedicellariæ are rather small 

 (ca. 0(S mm. head); the blade is short and broad, the edge irregularly sinuate and finely 

 serrate (PI. VI. F'igs. 39, 40. PI. VII. Fig. 1). In the larger forms the valves join only 

 in the outer part, in the smaller ones in almost their whole length, and in these the 

 edge is almost quite straight ; the larger ones have a well developed meshwork in 

 the bottom of the blade. The ophicephalous (PI. VI. Fig. 4) and the triphyllous 

 pedicellariæ (PI. VI. Fig. 23) do not present any marked peculiarities. — The spicules 

 of the tubefeet are bihamate and usually very few in number. 



The buccal membrane contains numerous small, irregular, fenestrated plates 

 inside the buccal plates; outside these there are some few thick plates opposite to 

 the gills; sometimes there are numerous small plates over almost the whole mem- 

 brane opposite to the interambulacra, whereas there are almost none opposite to 

 the ambulacra. According to de Loriol (Op. cit. p. 21) these plates „portent de soies 

 d'une grande linesse"; these are certainly stalks of pedicellariæ, which occur (in 

 spite of DE Loriol's assertion that he does not find pedicellariæ on the buccal mem- 

 brane) on the large plates at the gills and on the buccal plates, but not on the 

 other small plates, when such are developed. Bihamate spicules are not numerous 

 in the buccal membrane; sometimes, however, small patches are found quite studded 

 with spicules. The gills contain the usual irregular plates, but mostly in the lower- 

 part only; in the finer branches only bihamate spicules are found. — The walls of 

 the intestine are provided with rather few bihamate spicules, whereas the stone- 

 canal is quite studded with such spicules, interwoven with each other. The genital 

 organs consist of a great many long, unbranched tubes, arranged round a long 

 main stem; the latter and the lower part of the tubes are quite full of bihamate 

 spicules, whereas the tubes are quite destitute of spicules. 



Many specimens of different sizes of this most beautiful sea-urchin were 

 taken at Koh Kram, 20 — 30 fathoms; also some specimens at Koh Chuen, 30 faths., 

 and between Koh Kahdat and Koh Kut, 8 — 10 fathoms, all on hard bottom (sand, 

 shells, stonesj. A specimen of only 2"5 mm. in diameter is already quite typical, 

 with both kinds of globiferous pedicellariæ developed. One specimen is abnormal 

 and deformed, one of the ambulacra reaching only halfway to the apical system. 



Several of the specimens are infested with a small parasitic Gastropod, 

 probably an Eulimid. Further I observed on several of them a little crab, which 

 devours the spines (probably the muscles only), pedicellariæ and tube feet, quite 

 cleaning the test; it mostly follows one area from the top downwards. Where it 

 has been, one may find young stages of pedicellariæ in enormous numbers, quite 

 covering the test; also young spines may be found in such places. If the lube feet 

 are regenerated, I cannot ascertain beyond doubt, but il seems so. 



