CLYPEASTRINA. 15 



with four valves and these quadridentate pedicellariae might perhaps be considered 

 the most specialized form occurring in the suborder. In the Laganidae also, 

 pedicellariae of three kinds occur and in some species are quite common and the 

 same seems to be true of the Fibulariidae, so far as known. In Arachnoides, 

 pedicellariae are excessively rare, some fine specimens seeming to lack them alto- 

 gether. Those that do occur are all of one kind, small and with only two valves. 

 In the Scutellidae, we find a similar marked reduction in the number and size 

 of the pedicellariae. Only in one species of Echinodiscus have I found ophi- 

 cephalous pedicellariae, and in no other genus of the family. The tridentate 

 and triphyllous pedicellariae are very small, and commonly have but two valves. 

 The triphyllous are so small, it is only by the greatest care that they can be found. 

 The tridentate, or more properly the bidentate, in Echinarachnius parma show 

 the further degradational feature of a total lack of apophyses in the valves. 

 Taking all the facts into consideration it seems clear that the clypeastroids have 

 sprung from a stock, provided with ophicephalous, tridentate, and triphyllous 

 pedicellariae and it may be added that to no family of regular Echini do they 

 show a closer resemblance in these particulars than to the Saleniidae. Adapta- 

 tion to a .more or less subarenaceous life seems to have been the cause of a deg- 

 radational change in the pedicellariae so that as the test has become more and 

 more flat and discoidal, there has been first a loss of the ophicephalous pedicel- 

 lariae, and a reduction in number of all kinds, and this has been followed by a 

 reduction in the size and number of valves of the pedicellariae themselves, until 

 the condition is reached which is characteristic of Arachnoides, where the pedi- 

 cellariae are very scarce, all of one kind, very small and with only two valves. 

 Further reduction to complete extermination seems to occur in some individuals 

 of this genus. 



The ophicephalous pedicellariae of clypeastroids, when present at all, always 

 have three valves (PI. 123, fig. 17), of which one has a very large basal "loop," 

 the second has a moderate one and the third has little or none (PI. 123, figs 5, 6; 

 PI. 124, figs. 10-12). The shape of the loop shows great diversity but is of little 

 importance as a specific character. The blade (PL 123, fig. 4/ PI- 124, figs. 9, 81 ) 

 is equally variable in form but is of more taxonomic importance. The stalk of 

 these pedicellariae is longer than the head but is solid and rather stout, and is 

 hollowed at the top (PL 124, fig. 13) so that the loops actually set into the 

 cavity. The heads are always small, .10-.20 mm. long with the loops adding 

 about half as much again. 



The bidentate, tridentate, and quadridentate pedicellariae are the commonest 



