104 HAWAIIAN AND OTHER PACIFIC ECHINI. 



The slender tridentatc ijedicellarioe (PI. 51, figs. 3, 12) are commonly present 

 on all parts of the test, but are often infrequent and may be wholly wanting, 

 especially in young individuals. The heads are from .30 to 3 mm. long, and 

 the neck and stalk show an equally great diversity. The head is usually 

 shorter than the stalk, and the latter may be many times as long, but when 

 the head is greatly elongated the stalk may be actually shorter. The neck is 

 longest when the head is shortest, and is so short when the head is very long 

 that it is virtually absent. The valves (Pis. 50, fig. 16 ; 51, fig. l) of which there 

 are commonly three but sometimes four, are straight or very little curved, 

 usually compressed, especially at the base of the blade, but often more or less 

 flattened, and the margins are coarsely dentate, becoming more finely serrate 

 near tip. The valves are 3-6 times as long as the width of the basal part, or 

 6-15 times as long as the width of the blade. There is usually, but not 

 always, a calcareous network more or less developed in the basal half of the 

 blade, and it sometimes extends nearly to the tip. 



The stout tridentatc pedicellariaj (PI. 51, figs. ^, 13) which are found scattered 

 on the test in varying abundance and are often wholly wanting, when fully 

 developed are strikingly different from the slender tridentate, but it is not 

 possible to draw a sharp line between them, for they seem to intergrade, 

 particularly in the genus Diadema, where one species has only slender 

 tridentate and another has only stout tridentate, and others have forms 

 which might be called by either name. The stout tridentate do not show 

 as great a diversity of size as the slender ones, for the head is rarely less 

 than .50 ram., and seldom exceeds 1.50 mm. There is little or no nock and 

 the stalk is usually about as long as the head, though it may be shorter, or 

 in other cases very much longer. The valves are usually decidedly curved 

 and meet only at or near the tip. The margins are coarsely dentate or 

 serrate for a part or all of their length. The length of the valve is only 

 2-4 times the width of the basal part, and only 3-4 times the width of the 

 widest part of the plate, which is commonly near the tip. There is usually 

 little or no calcareous network in the blade, but it may be very largely 

 developed. 



The non-glandular ophicephalous pedicellarioe (PI. 51, figs. 5, 14) are very 

 variable in their occurrence, as they are frequently rare and often entirely 

 wanting. When present, they are more likely to be found actinally than 

 abactinally. The head is from .20 to .65 mm. long, while the stalk is 3-5 

 times that length ; there is no neck. The valves are usually provided with 



