16 HAWAIIAN AND OTHER PACIFIC ECHINI. 



forms, occurring in all the genera examined, although they are often very "few 

 and far between." In them all, the stalk usually about equals the head, though 

 it may be either longer or shorter, and the neck is relatively quite short. They 

 differ from each other chiefly, as the names used indicate, in the number of valves. 

 The quadridentate (PI. 123, figs. 10, 11) are the least common, having been found 

 only in certain species of Clypeaster. They are the largest pedicellariae of the 

 suborder, the valves sometimes exceeding a millimeter in length. The valves 

 are compressed and meet only at the tip. The tridentate (PI. 123, figs. 1, 29, PI. 

 124, fig. 1) are also often of large size, with valves exceeding a millimeter in 

 length, but they may be very small, not one tenth the size of the large ones. 

 They are found in all species of Clypeaster, and in the Laganidae, Fibulariidae, 

 and some ScuteUidae. The valves meet at or near the tip or for more or less 

 of their entire length; they are sometimes straight but more commonly are 

 curved to a greater or less degree. These pedicellariae show great diversity not 

 only in size and relative abundance, but in the form of the ^•alves. The latter 

 occasionally have "loops" as in ophicephalous pedicellariae, but the shape of 

 the blade never approaches the characteristic ophicephalous form. With the 

 triphyllous pedicellariae however, the tridentate show an evident intergradation 

 and the line between the two kinds is purely arbitrary. The bidentate pedicel- 

 lariae (PI. 125, figs. 2, 23) occur only in the Arachnoididae and ScuteUidae. 

 They do not occur with either quadridentate or tridentate. They are always 

 small, the valves never exceeding .50 mm. and being usually less than .30. The 

 two ^•alves meet only at or near the tips and are usually distinctly compressed. 

 They are of very diverse shapes and those of Echinarachnius are remarkable 

 for the absence of any apophysis, so that no distinction between base and blade 

 is feasible. 



The biphyllous and triphyllous pedicellariae differ from the bidentate and 

 tridentate chiefly in being smaller and in having the valves of more bizarre 

 shape and in usually having a very much longer stalk and longer neck. They 

 differ from each other only in the number of valves. The biphyllous (PL 125, 

 fig. 7) have the valves usually somewhat compressed but meeting broadly at 

 the tip; they are only .05-. 10 nmi. in length. They are found only in the more 

 speciahzed ScuteUidae. The triphijUous, on the other hand, occur in the less 

 specialized ScuteUidae and in all the other families except the Arachnoididae. 

 The valves are no larger than those of the biphyllous and because of their very 

 small size they are often exceedingly hard to find. The blade is generally 

 somewhat expanded and its margin may be finely serrate (PI. 123; figs. 24, 25) or 

 dentate (PI. 124, figs. 20, 23). 



