106 HAWAIIAN AND OTHER PACIFIC ECHINI. 



fig. 16), while at other times thej are very large on the stalk but hardly 

 noticeable on the valves. As a general rule one may say that if they are 

 conspicuous on the valves, they will be small on the stalk, and vice versa. 

 The stalk is greatly expanded and more or less concave at the top (PL 51, 

 fig. 20). 



In all pedicellariaB in this family the stalk consists of a calcareous rod 

 more or less pitted or fenestrated, at least at the enlarged ends. In its 

 simplest and slenderest condition the rod is nearly smooth and cylindrical 

 for the great part of its length, but is more or less abruptly and conspicu- 

 ously enlarged both at the base and at the top ; the enlarged portions are 

 rough, rather distinctly ridged, with cross-bars and pits or openings between 

 the ridges. In other cases the rod is rough with projections, and it may be 

 somewhat flattened and regularly fenestrated in such a way as to appear 

 ladder-like, as though it were made up of two rods united by cross-pieces. 

 Where still more developed the rod is much thicker and is ridged for its 

 whole length with cross-bars, perforations, or pits in between the ridges, and 

 the enlargement of the base and top are relatively small. In its extreme 

 development the stalk has eight or more ridges, and may appear as though 

 made up of that number of rods, closely united by cross-bars, particularly at 

 the two ends. In Micropyga the component parts of the stalk are very 

 slender, and the connections between them few and widely separated ; such 

 a stalk is best described as made up of numerous calcareous threads, closely 

 united at the ends but only loosely and irregularly connected with each 

 other elsewhere. While at first sight such a stalk appears quite different 

 from that found in other Diadematidoe, the difference is, after all, in degree 

 rather than in kind. 



The sphoeridia (PI. 50, fig. 20) are more or less ellipsoidal, rarely globular 

 bodies, attached to the surface of the test and seldom sunken in depression.s 

 or pits of any kind. They are very small (.15-.30 mm. long), and are provided 

 with short stalks which connect them with minute tubercles on the plates. 

 They are confined to the ambulacra and are generally rather numerous, 

 from 3 to 12, or even more, occurring in each ambulacrum. They are most 

 common on the actinal surface, especially near the peristome, but in some 

 cases they occur only near the ambHus, and in others they are found all 

 along the ambulacnnn, from ]Kuistome to ocular plate. They most com- 

 monly occur near one of the large tubercles, and usually at its outer side, 

 so that they frequently form a vortical series between the large tubercles 



