ECHINOIDEA. I. 



79 



the spine (PL XIV. Fig. 30), being placed like a cap on the point.) — It cannot be decided, whether the 

 spines of the peristome are placed in concentric circles, but I think it probable. On the abactinal side 

 the rather numerous primary spines are irregularly scattered over the whole surface, not arranged in 

 series (Figs. 5 — 6). A great many miliary tubercles carrying small spines or pedicellarise , are scattered 

 over as well the ambulacral as the interambulacral plates. 



The structure of the spines is as usual. The small ones are regular, perforated tubes ending 

 in a fine point; no thorns seem to be found on them. The large spines with the hoofs are constructed 

 in a more complicated manner. The longitudinal ridges are very prominent, narrow, widened in the 

 outer end, and a little hollow on the outside; in transverse sections they are T-shajjed. Between these 

 ridges connecting beams are often developed, so that a rather complicated reticulation is formed; 

 towards the central hollow the boinidary is regular. The small abactinal spines have little conspicuous 

 longitudinal ridges, not widened along the outer surface (PI. XL Fig. 2, a — c). 



The apical area resembles that oi Hygroso)na luciilnitnni^ which has been figured by Agassiz 

 (Chall. Ech. PI. X. a. Fig. 3); but the form of the plates is otherwise only seen with difficulty. 



The tube feet are placed in one irregular series on the actinal side; on the abactinal side they 

 are placed alternally two opposite each other, and one single, as is shown by the pores in Fig. 5; most 

 frequently the inner one of the two placed at the same height (the one in the inner accessory ambu- 

 lacral plate) is somewhat larger than the others. The spicules are irregular, net-shaped plates; they 

 may be exceedingly complicated, and are not arranged in longitudinal series, but inclose the whole 

 foot. They are placed in 2 — 3 layers; in the tube feet of the abactinal side the inmost layer consists 

 of larger, perforated plates, the outermost one of irregularly branched spicules (PI. XI. Fig. 13), in the 

 tube feet of the actinal side the whole thing forms a complete confusion of net-shaped plates. No 

 sucking disk is developed. 



The sphaeridia; (PI. XIV. Fig. 12) are of the common form, and, as is commonly the case in 

 the Echinothurids, are placed along the series of tube feet quite up on the abactinal side. 



The pedicellarife : The tridentate pedicellarite occur in two different forms, not, however, 

 sharply distinguished. In the larger form (PI. XII. Fig. 41, PL XIV. Fig. 21), the head of which reaches 

 a length of up to 3-5""", the blade is filled by a very complicated net of meshes rising into strong 

 thorns, partly arranged in series; it is somewhat widened in the point, more narrow in the middle, 

 but the edges, which are here coarsely serrate, are not involuted. The valves are rather wide apart, 

 when the pedicellaria is shut. The neck is ver>- short, the stalk of the common structure. In the 

 smaller form the blade is almost of the same breadth throughout its whole length, not widened 

 in the point; it resembles very much the form found in Phormosoma placenta — which is, no doubt, 

 as well the most frequent as the simplest form of tridentate pedicellarise in the Echinothurids — but 

 the widenings of the upper end of the apophysis reach quite to the edge of the blade, they do not 

 end down on the side as in Ph. placenta. In the bottom of the blade there is a not very much devel- 

 oped reticulation, in the smallest ones almost none is found (PL XII. Fig. 22), in the larger (PI. XIV. 

 Fig. 16) it is more developed, in the largest ones even with a short, prominent, serrate crest, thus 

 forming a transition to the large form. In the small ones the valves join completely, when the pedi- 

 cellaria is shut; the edge is finely serrate; the neck is rather long, the stalk of the common structure. 



