46 ECHINOIDEA. I. 



so that there can be no doubt that it is a genuine ophicephalous pedicellaria. It is a highly curious 

 fact that each of these three kinds of pedicellarise, two of which show a very perfect development, are 

 only found in a single genus, while none of the other Echinothurids seem to have a corresponding 

 form of pedicellarise. 



The tridentate pedicellarise are very richly developed in the Echinothurids. Most frequently 

 their form is simple; the valves are leaf-shaped, and the blade is more or less filled by a net of meshes 

 which may be very spinous. In another common form the edges of the blade are involuted, so that 

 only the point of the blade is somewhat widened; in this form the blade is commonly strongly bent, 

 so that the valves are widely separated, and only join with their points when the pedicellaria is closed. 

 Both these forms may be found in the same species; and in a group of species, A. varium and the 

 species most nearly allied to it, even three different kinds of tridentate pedicellarise are found, viz. 

 besides the two mentioned forms a short, broad one with coarsely serrate edge (PI. VIII, Figs. 4, 27). 

 A peculiar short and broad form is found in Ph. luculcntum; it recalls to some degree an ophice- 

 phalous pedicellaria, but as it has no indication of an arc, there can scarcely be any question of inter- 

 preting it as any thing else than a form of the tridentate pedicellarise. The tridentate pedicellarise 

 may be very large, especially those with involuted edge; these have commonly a very short neck. 



The triphyllous pedicellarise (PL XII, PI. XIII. Fig. 23) are very well developed in the Eclrino- 

 trmrids; peculiar to these in comparison with the triphyllous pedicellarise of the Echinids is the fact 

 that the upper edge of the apophysis spreads over the lower part of the blade, and continues up along 

 its sides; in some, for instance Ph. placenta, this cover-plate is not much developed, in most species 

 it is highly developed, and covers a great part of the blade. Generally there are then some large 

 holes in the median line, and some smaller holes around ; the part continuing upward along the lateral 

 edges of the blade, is most frequently without holes. The upper edge of the blade is generally finely 

 serrate. The holes in the blade are always placed in rather regular curves from the middle obliquely 

 upward on either side. -- The peculiar bottle-shaped, two-valved pedicellaria, figured by Agassiz from 

 Phormosoma tenue (Chall. Echinoidea. PI. XLJV. Fig. 21) is presumably an abnormal form. I have 

 examined a couple of the type specimens, but have only found the common, three-valved form. 

 Agassiz (Chall. Echinoidea. p. 84) thinks that this bottleshaped pedicellaria is only a modification of 

 the ordinary type of pedicellarise, in which the terminal edge becomes raised to form a spoon-shaped 

 valve. This is absolutely wrong; one form is a triphyllous pedicellaria, the other a tridentate one. 



The stalk of the pedicellarise in by far the greatest number of Echinothurids is thin, irregularly 

 perforated, not distinctly tube-shaped (PI. XIV. Fig. 31). In the large tridentate pedicellarise, as in A. 

 varium, also the stalk is somewhat coarser; the stalk of the ophicephalous pedicellariae of Tromikosotna 

 is a rather thick tube. In Ph. asterias the construction of the stalk is quite exceptional among the 

 Echinothurids; it consists of some long, very thin calcareous threads, only united at the ends of the 

 stalk, at most connected in the intervening part by quite few transverse ridges. 



Also the inner anatomical structure seems to yield good systematic characters. Thus Bell 

 (Catalogue p. 142) mentions as a chief difference between the genera Phormosoma and AstJicnosoma that 

 the latter has highly developed longitudinal muscles* dividing the body-cavity into chambers, while 

 such muscles are wanting in Phormosoma. - To this, however, is to be remarked that the specimens 



