68 ECHINOIDEA. I. 



are any spicules found in the skin on the outside of the plates (which may easily be prepared off) or 

 in the bag of skin round the spines of the actiiial side. 



The pedicellarise. The trideiitate pedicellarise occur only in one form, with simply leaf-shaped 

 valves. The size is very different, from quite small ones to such where the head has a length of 2 mm . 

 The form of the valves is rather varying, sometimes short, broad, and flat, almost without any net of 

 meshes, sometimes long, narrow, and deep, or long and broad, with a rather well developed net of 

 meshes at the bottom. On PI. XII. Figs. 2, 3, 7, 26, 37, 39 some forms are given; all transitions between 

 them are found; but narrow and broad forms do not seem to occur in the same individual, as in Ph. 

 bursarium. The upper end of the apophysis is widened, but these widenings do not reach to the edge 

 of the blade, they cease about midway on the side. Also the net of meshes at the bottom of the blade 

 is an immediate continuation of the upper end of the apophysis; it is always smooth. The sides of 

 the blade are most frequently a little bent outward, especially on the narrow forms. The edge is 

 finely serrate, which is only to be seen under higher magnifying powers. The trideiitate pedicellaria 

 figured on PI. XIII. Fig. 7, is the long, narrow form. The neck is rather long, the stalk is thin, irre- 

 gularly perforated. 



In some specimens from st 40 the trideiitate pedicellarise are especially long and narrow (the 

 pedicellaria figured on PI. XIII. Fig. 7 is one of these), so that we might be inclined to regard these 

 as a separate species or variety. As there seems, however, to be no other characters, -- with the 

 exception that the tube feet of the actinal side are more rudimentary than usual -- and as the form 

 of the pedicellariee may be rather varying, there can scarcely be any question of regarding these speci- 

 mens otherwise than as good Ph. placenta. 



The triphyllous pedicellarise have been excellently figured by Wyv. Thomson (PI. LXII. Fig. 6), 

 so I only figure one valve seen from the inside (PI. XII. Fig. 21). The cover-plate is here very slightly 

 developed, but in this feature some variation is found. The outer edge is finely serrate. 



Sometimes two-valved pedicellarise are found, especially trideiitate ones, more rarely triphyllous 

 ones. They are constructed as the normal three- valved pedicellariee, and have an apophysis as these, 

 only more slightly developed. It is rather interesting to compare these pedicellarise with the normally 

 two-valved ones in Porocidaris; in the latter the apophysis is quite wanting. I have found a few 

 instances of a tridentate pedicellaria, in which the edge of the blade was a little involuted for a short 

 space below, so that it reminded of the small tridentate pedicellarise in Ar&osoma fenestratum. 



The spheeridise (PI. XII. Figs. 23, 25) are commonly almost globular, but show too great varia- 

 tion to be reliable specific characters. As observed by Agassiz they are placed in a series along the 

 tube feet from the mouth far up on the abactinal side. 



According to Bell (69. p. 438) the longitudinal muscles are caltogether absent from Phormo- 

 soma. I cannot agree with Bell in this statement; they are also found in Ph. placenta, and are of 

 the common form, but they are fine and break easily, so that the preparation must be made with 

 great caution, in order to get a distinct view of them. I think it only little probable that any greater 

 individual variation with regard to the development of the longitudinal muscles should be found in 

 Ph. placenta, so that they might even sometimes be quite wanting. The organs of Stewart, as shown 

 by Bell (op. cit), are very little developed. 



