BCHINOIDEA. n. I2 c 



single globiferous pedicellaria, resembling those of Schizaster Philippii, in a young specimen from the 

 Mediterranean. 



The tridentate pedicellariae occur, as lias been said already, in two distinct forms, one with 

 elongate, slender valves, the other with short and robust valves. The slender form occurs in very 

 different sizes, from ca. cr2 mm to ca. 2 mm (length of head). The shape of the head is well seen in 

 Perrier's PI. VII. 4. a. The valves (PI. XVI. Figs. 1, 9) are long and narrow, widely apart, joining only 

 at the point which is a little widened, spoonshaped, with the edges finely and closely serrate. The 

 edge of the lower, narrow part of the blade is more or less coarsely serrate, but it may sometimes be 

 quite smooth. The bottom of the blade is abruptly deepened in a narrow stripe along the median line, 

 with some crossbeams passing over it. In side view this deepening is seen as a narrow crest along 

 the back of the blade, in dorsal view of the blade it is seen as a sharply defined longitudinal keel, 

 formed by two knotted edges. The basal part is remarkably narrow; the apophysis is large, mostly with 

 smooth edge. The three points looking downwards from the basal part, mentioned and figured by 

 Perrier, I have never seen. 



In smaller specimens of this kind of pedicellaria; the valves join to a larger extent, in quite 

 small ones they join in their whole length. The blade is comparatively broad, simply leafshaped 

 (PL XVI. Fig. 27). All transitional forms are found between the largest and the smallest specimens, as 

 is very well shown in the figures given by Doderlein. Two-valved specimens sometimes occur. 

 The neck is well developed, though rather short in the largest specimens. The stalk is an irregularly 

 fenestrated tube, with a small milled ring at the lower end for the attachment of the muscles, just 

 as in the spines, only, of course, much more feebly developed. Such a ring is found on the stalk of 

 all the pedicellariae except the ophicephalous ones. 



The second form of tridentate pedicellariae (PL XVI. Fig. 8) is much coarser, with a thick head 

 and a short neck. The valves (PL XVI. Figs. 7, 10) are much narrowed in the middle, but the basal part 

 passes evenly into the blade (a rather conspicuous difference from Macropneustes spatangoides, (comp. 

 p. 128. PL XVI. Figs. 3, 13). The edge of the outer part of the blade makes an obtuse angle with the 

 narrowed part; it is finely serrate. The point of the blade is generally somewhat produced inwards. 

 There is a more or less developed meshwork in the lower part of the blade. The dorsal side of the 

 blade is uneven, knotted (PL XVI. Fig. 10). In larger specimens of this kind of pedicellariae the nar- 

 rowed median part of the blade may be rather long (PL XVI. Fig. 25), such valves looking more like 

 usual tridentate pedicellariae. Perrier (Op. cit. p. 278) 1 names this kind ophicephalous pedicellariae in 

 spite of the fact that no bow is found below the valves. Now, to be sure, it may well be maintained 

 that it is no absolutely necessary criterion for ophicephalous pedicellariae that these arcs must be 

 present (see also de Meijere. Siboga-Ech. p. 244— 45) — as well as, on the other hand, that such arcs 

 may occur also on undoubtedly tridentate pedicellariae, as has been shown both by de Meijere and 

 by mvself. In this case, however, it cannot be doubted that these pedicellariae are tridentate and not 

 ophicephalous, because true ophicephalous pedicellariae of quite typical structure are also found. — 



1 At this place reference is made to a figure of a large tridentate pedicellaria (PI. VII. 4. a), but the text and the 

 explanation of the plates leave no doubt that the Fig. 4- b is meant, which evidently represents a pedicellaria of this second 

 tridentate form. 



