ECHINID^. 237 



simple to the most specialized form takes place are easily followed, although as a 

 rule the form found in any one species is very constant. In some species how- 

 ever, like P. magellanicus, considerable diversity is found even in one specimen. 

 Aside from the great differences shown in the actual size of the valves, there is 

 much variation in the relative length and breadth of the blade, the base and the 

 terminal tooth. In many species the tissue surrounding the valves is more or 

 less filled with calcareous spicules, which may be either dumb-bell shaped, or 

 bluntly, or sharply bihamate. 



The tridentate pedicellarise reveal as great a diversity of structure as that 

 shown by the globiferous, but the diversity is m.uch less correlated with specific 

 limits and it is difficult to detect anything like progressive speciahzation. The 

 simplest condition of the valves is probably that shown by many of the small 

 ones, where the blade is about twice as long as the base, perhaps a httle more 

 than twice its own width, only slightly curved and roundly pointed at the tips. 

 From this simple condition, specialization has diverged, on the one hand towards 

 excessively elongated, compressed valves, and on the other towards broad, 

 stout valves little compressed. Several forms are often found on one specimen, 

 but the two extremes do not occur together. The amount of calcareous mesh- 

 work in the cavity of the blade is variable; sometimes it is almost wholly wanting 

 while in other cases it occupies nearly the entire inner surface of the blade. 

 These pedicellariaj have a slender stalk, and usually more or less of a neck. The 

 valves themselves may be more than three millimeters in length but are usually 

 about a miUimeter, though they are often much less. It is an interesting fact 

 that the globiferous and tridentate pedicellarise seem to be more or less supple- 

 mentary to each other, for when one kind is unusually abundant, the other is 

 often quite wanting. Thus in some specimens, even to some extent in certain 

 species, tridentate pedicellariai are common enough, but we search in vain for 

 the globiferous, while in others tridentates are not found but the globiferous 

 occur in sufficient quantity. 



The ophicephalous pedicellariae are always present in greater or less numbers 

 and although ordinarily easily recognized, they may intergrade more or less 

 with the tridentate. They have no neck but the valves have well-developed 

 "articular loops," which differ strikingly in size on the three valves. The 

 stalk is thick and solid. In the form of the valves two quite distinct types occur, 

 the unconstricted and the constricted. In the former there is no sharp distinc- 

 tion between base and blade, and the valves are more or less triangular: they 

 are sometimes narrow and elongated and then intergrade very naturally with 



