15 



pedicellariæ as in D. saxatile. Besides the usual claviforni pedicellariæ true ophice- 

 phalous pedicellariæ also occur; in very young specimens I have found only tlie latter 

 form; in a specimen of 10 mm. diameter I have found both forms together; in 

 larger specimens only the claviform ones seem to occur. The ophicephalous pedi- 

 cellariæ are long-stalked without neck and without glands on the stalk. The valves 

 (PI. III. Fig. 16) are small but of the typical structure, with the usual arc below the 

 articular surface. The tridentate pedicellariæ (PI. III. Fig. 1, PI. IV. Fig. 28) are very 

 dilferent from those of D. saxatile; the valves are rather short (head c. 1 mm), the 

 blade is broad, spoonshaped, with rather few serrations on the edge, and they are wide 

 apart, joining only at the point. These tridentate pedicellariæ are found as well on 

 the test as on the buccal membrane ; on the test some larger tridentate pedicellariæ 

 (head 1,3 mm.) maybe found, with narrower, simply leafshaped blades. These latter 

 are more similar to those of D. saxatile, though different enough, especially in the 

 edge being much less seri'ate. The small tridentate pedicellariæ (PI. III. Fig. 10, 

 PI. V. Fig. 13) are more like those of Ü. saxatile, only mostly broader. In some 

 small specimens (14 — 15 mm. in diameter) only small tridentate pedicellaries were 

 found. The spicules are like those of Ü. saxatile; they are found only in the outer 

 end of the actinal tubefeet ; in the abactinal ones neither common spicules nor 

 supporting beams in the partition wall are found. 



D. iiiexicaiiiiui Ag. As an essential character of this species Agassiz points 

 out (Revision p. 408) that the abactinal system is larger in proportion than in D. 

 saxatile, with a larger anal system and larger genital plates. There is, indeed, 

 some difference in the size of the abactinal system in the species of Diadema, but 

 since Agassiz's D. setosum contains both D. saxatile and D. antillaruin, his statement 

 must be corrected. The diameter of the apical system is, contrary to the statement 

 of Agassiz, larger in D. saxatile than in the other species; in D. antillaruin it is of 

 about the same size as in D. mexicanum. The single measurement given by Agassiz 

 for the abactinal system of D. setosum (Revision, p. 275) has probably been taken 

 from a specimen of Ü. antillarum. I shall give some measurements of the three 

 species named, to which D. Sauignyi may be added. 



In the form and size of the apical plates and in the size of the periproct 

 and the peristome I do not find any reliable difference. The pore-areas are as in 

 Ü. antillarum; the pores are not much smaller towards the peristome. There are 

 no dark impressions on the genital plates. The naked space in the interanibulacral 



