178 HAWAIIAN AND OTHER PACIFIC ECHINI. 



half of the plate. The ocular plates are somewhat pentagonal, with very 

 small pores. The anal system is large, 18 mm. across, and is covered by 

 several concentric circles of small plates, the inner ones the smallest ; many 

 of the outer ones carry one or more secondary spines. 



The smallest specimen of thetidis which we have is 72 mm. across, with 

 the actinal system 18 mm., the abactinal system 11 mm., and the anal 

 system 8 mm. in diameter. The interambulacra are 26 mm. wide at the 

 ambitus and have 32 plates in each half-column, while the ambulacra are 20 

 mm. wide and are made up of 43 pairs of plates. The primary tubercles of 

 the abactinal side are about as numerous as in large specimens and there- 

 fore appear much more numerous relatively. Actinally they are fewer and 

 tend to form a regular marginal row along the outer border of each inter- 

 ambulacrum, a much less regular series at the inner ends of the same plates, 

 and two very irregular series in the middle of each ambulacrum. 



The spines of this species offer no peculiarities and the pedicellaria? are 

 very much like those of Owstoni and bicolor. We have not found any 

 " dactylous " ones, however, although careful examination has been made of 

 several specimens. The tridentate pedicellarire are exceedingly abundant, 

 particularly just below the ambitus, while the triphyllous are less common. 



The tridentate pedicellarise (PI. 66, figs. 15, 17) are extraordinarily diver- 

 sified in size and form, though on the same general plan. The heads are 

 thick and blunt, and the stalks are about twice as long as the head or longer. 

 The valves (PI. 66, figs. 6-12) range in length from one-fifth of a millimeter 

 to over two millimeters ; they are in contact for nearly their entire length, 

 except in rare cases, where only the terminal halves touch. In small pedi- 

 cellariae, the valves have a nearly straight or somewhat convex, smooth 

 margin, but in the larger ones it is more and more sinuate, until in the 

 largest it is very coarsely toothed. In all large pedicellariae the blade is 

 filled with a coarse mesh-work which may rise up into irregular serrate 

 ridges (PI. 66, figs. 11 and 12) ; in large valves the tip may be very strongly 

 hooked. 



The triphyllous pedicellarice (PI. 66, fig. 16) are not very numerous and 

 have rather elongated heads on slender stalks; the valves (fig. 17) are 

 narrow, with a very long, perforated cover-plate, and rounded at the tip. 

 The calcareous spicules (PI. 66, fig. 13) in the tube-feet are small, irregular, but 

 essentially triradiate, bodies, sometimes appearing as small perforated plates. 



