ECHINOIDEA. I. 53 



that it is a normal form of pedicellariæ cliaracteristic of tliis group of species. Of tridentate pedicel- 

 lariæ two kinds are fonnd. The larger form has not been seen by W y v. Thomson, but I have 

 found it on a fragment kept in British ]Museum under the name of v.Calveria Phoriiiosoma^^ but being 

 nndoubtedh- an original specimen of Wyv. Thomson's Calvcria fencstrafa. The edges of the blade 

 are much involuted, only the point is widened and deeply indented in the edge (PL XIV. Fig. 32). The 

 valves are highly curved outward, so that they are wide apart when the pedicellaria is shut. The 

 length of the head up to 2™'". The other form is very varying according to its size (PI. XIV. Figs. 8, 

 17, 18, 24). Larger specimens recall to sonre degree the large form, but the widened part of the blade is 

 comparatively larger, the involuted part smaller; the edge of the widened part is coarsely sinuate. When 

 the pedicellaria is shut the valves are only a little apart (the figure by Wyv. Thomson. PI. LXVII, 7). 

 In the very smallest ones only a quite small space below is involuted, and the edge of the upper 

 part is quite straight. All transitions between these forms are found, so that they can only be inter- 

 preted as modifications of one kind. Their neck is short, the stalk of the common structure. The 

 triphyllous pedicellariæ have the cover-plate much developed, and are lengthened and narrow; the 

 edge finely serrate (PL XII. Fig. 33). — The primary spines on the actinal side are curved and end in 

 a little hoof. The tube feet as in 6". Iiystrix arranged in three separated series; the spicules large, 

 irregular fenestrated piates, in the lower part of the tube foot arranged in four separated series; the 

 sucking disk well developed. As characteristic of this .species Wyv. Thomson lays stress on the 

 large membranous interpaces between the piates; as Bell (72) has shown that this feature is very 

 varying this character is not reliable. For the present there is no other sure character than the pedi- 

 cellariæ, and even if the tetradactylous ones be wanting, which seems most frequently to be the case, 

 be it now that they have fallen off, or perhaps ma}- be quite wanting in some individuals, the tri- 

 dentate pedicellariæ are sufficiently characteristic, so that no confounding can take place between this 

 species and Cak'cria hystrix. A separate genus must be formed for this species; I propose the name 

 of Aræosoma'). — No doubt it is this .species that Agassiz (6) described as ^[sf/iaiosoiiia Riyiioldsii] 

 but later (9) retired as a synonym of A. hystrix. 



To this genus will further have to be referred A. coriaceum Ag. Of this species I have 

 examined a specimen from Chall. st. 169. This station is not enumerated by Agassiz as a locality of 

 the species, but according to the statement of Prof. Bell the determination of the aniraal has been 

 made by Agassiz, .so that it may be taken to be due to an omission that this .station has not come 

 in. — The tetradyctylous pedicellariæ agree exactly with those of A. fcncsfraficin, so that no specific 

 difference seems to be found in this structure. They were only found on the upper side, and only a 

 few ones, as it was almost rubbed off. Of the tridentate pedicellariæ I have not found the largest 

 form. The smaller form (PL XIV. Fig. 5) is especially highly developed, the head up to 2'""' long. The 

 blade is filled by a very complicated net of nieshes, more developed than in A. faicstratitiii. As in 

 this latter, forms are also here found with almost straight edge, as well as such as are rather similar 

 to the large involuted form, and all transitions between them. Triphyllous pedicellariæ chiefly of the 

 same form as in A. /enestraf n 11/ (PL XII. Fig. 27). (The form figured of A. fcncstratiim with the cover- 

 plate open in the median line, is not constant; they are as commonly found with the projections 



■) åpati'iq — thin. 



