144 HAWAIIAN AND OTHER PACIFIC ECHINI. 



viduals. The length of stalk is also very variable, though it commonly 

 exceeds that of the head. The structure of the stalk is of the usual type 

 in the Diadematidte, a cylinder of irregularly but closely united coarse rods. 

 It is very interesting to note that in Kamptosoma the stalk is made up 

 of "long threads, almost only united at the ends" (Mortensen); the stalk 

 in this genus, therefore, bears the same relation to the stalk in other Echino- 

 thurids that the stalk of the pedicellarioe in Micropyga does to that of the 

 other Diadematids, — a very notable case of " parallelism." The valves of 

 the tridentate pedicellarioe vary so much in size and form that a general de- 

 scription of them is impracticable, but certain kinds are very constant and 

 easily recognizable, and these deserve a few words. First of all are involute 

 valves, which are more or less curved and meet only at tip (PI. 66, fig. 1) ; 

 in these the margins of the blade are rolled in, forming a nearly cylindri- 

 cal middle part of greater or less length between the base of the blade and 

 the somewhat expanded tip ; secondly, there are contiguous valves, which 

 are straight and in contact for nearly their entire length, — these may be 

 very broad (PI. 65, fig. 18), or rather narrow (PI. 64, fig. 5) ; a third sort 

 may be referred to as convergent valves, which are straight or little curved, 

 little or not at all involute, meeting only at tip or for their distal half ; such 

 valves are shown on PI. 65, fig. 5 ; see also PL 67, figs, b, 12. Although 

 these three sorts of valves are usually recognizable with ease, they do inter- 

 grade more or less (PI. 65, fig. 4; 66, fig. 2). The ophicephalous pedicellaria} 

 (PI. 65, figs. 1, 2) are much less common, as a rule, than the tridentate, 

 and have been found as yet in only a comparatively few species ; they are 

 strongly constricted at the middle, and the articular loops on the valves 

 (PI. 65, fig. 3 ; 67, fig. 21) are well developed ; these pedicellaria? show no 

 tendency to intergrade with the other forms. The fourth form of pedicellaria 

 was first described by Wyville Thomson as a " tetradactyle " pedicellaria, 

 and Mortensen has retained the name. Unfortunately, however, the num- 

 ber of valves is often three or five, so that the " tetra" — is very inappro- 

 priate ; we would suggest that this form be called simply the dactylous 

 pedicellarise. They are known only from certain species of Araaosoma, 

 and show considerable diversity in the form of the valves ; these may be 

 long and slender, united only near the base and quite free distally (PI. 66, 

 fig. 20), or they may be much shorter and well surrounded by tissue as 

 de Meijere figures them, or the valves may be wholly imbedded in tissue 

 (PL 64, fig. 1). The valves differ notably in shape (compare PL 64, fig. 4, and 



