ECHINOIDIiA. II. I2C 



single globiferoiis pedicellaria, resembling those of ScJiizastcr Philippii, in a yonng specimen froin the 

 Mediterranean. 



The tridentate pedicellariæ occnr, as has been said already, in two distinct foiins, one with 

 elongate, slender valves, the other with short and robnst valves. Tlie slender form occnrs in very 

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

 Perrier's PI. VII. 4. a. The valves (PL XVI. Fig.s. 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. Tiie 

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

 quite smooth. The bottom of the blade is abruptly deepened in a narrow stripe along tlie 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, mentioued and fignred by 

 Per rier, I have never seen. 



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

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

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

 is very well shown in the figures given by D 6 d e r 1 e i n. 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, nnich more feebly developed. Such a ring is found on the stalk of 

 all the pedicellariæ except the ophicephalous ones. 



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

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

 passes evenly into the blade (a rather conspicuous difference from Alacropnetistcs spataiigoides, (comp. 

 p. 128. PI. 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 tlie 

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

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

 usual tridentate pedicellariæ. Perrier (Op. cit. p. 278)' names this kind ophicephalous pedicellariæ in 

 spite of the faet 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 pedicellariæ that these arcs must be 

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

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

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

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



■ At this place reference is made to a figure of a large tridentate pedicellaria IPI. VII. 4. al, but the text and the 

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

 tridentate form. 



