64 HAWAIIAN AND OTHER PACIFIC ECHINI. 



larioB is comparatively slight, except in the matter of size, and it seems 

 possible to find good generic, and even specific, characters in the form of the 

 valves. In view of the difficulty, if not impossibility, of doing this in the Cida- 

 ridae, it may seem very improbable that it can be done in the Arbaciadce ; but a 

 little consideration will make clear the difference between the two families. 

 In the Cidaridse, the spines, as well as all other outgrowths from the test, show a 

 remarkable diversity of form even in a single species, and individual differences 

 are very great ; it is not strange, therefore, that little reliance can be placed 

 on the pedicellariaB for purposes of classification. In the Arbaciadae, on the 

 other hand, the spines are remarkably constant in form in any given species, 

 and it is not strange, therefore, that the pedicellarise show a similar constancy. 

 It is interesting to note further that the longest and slenderest tridentate 

 pedicellarisB occur in that genus (Coelopleurus) which has the longest and 

 slenderest spines, while those genera (Habrocidaris, Podocidaris) which have 

 short flattened spines have the valves of the tridentate pedicellarioe short 

 and flat. 



The ophicephalous pedicellarioe (PL 47, fig. 2) are almost invariably 

 abundant, especially on the actinal surface, but their abundance shows con- 

 siderable variability in different individuals. They are always to be found, 

 and usually in numbers, on the buccal plates and on the ambulacral plates of 

 the peristome, and the whole ambulacrum, even up to the ocular plate, is 

 sometimes thickly covered with them. The heads vary in lengtli from .20 to 

 .75 mm., but there is comparatively little diversity in any one individual. 

 The stalks, on the other hand, are exceedingly variable in length, ranging 

 from tliree to twenty times the length of the head. The valves rest almost 

 directly on the head of the stalk, no " neck " being present, and the upper 

 end of the stalk is accordingly flattened or even concave. The exact form of 

 this upper end is very variable, though fairly constant within specific limits. 

 The valves are always provided with a conspicuous articular loop, and as 

 this varies greatly in size on the three valves, we can readily distinguish be- 

 tween valves a, h, and c (PI. 47, figs. 7-0). In a the loop is largest, in c it 

 is smallest. In their naturnl position the loop of b overlies that of c, while 

 that of (( overlies botli. The blade of tiie pcdicellaria is not appreciably 

 modified by tiiis striking dilTerence in llio basal part. Although the apo- 

 physis always forks at the base of tlie blade, one branch merging into each 

 margin, a portion of it often continues nearly to the tip as a more or less 

 conspicuous ridge, which is usually accompanied by a group of coarse cal- 



