238 HAWAIIAN AND OTHER PACIFIC ECHINI. 



the tridentate through the suppression of the loop, which is in any case rather 

 small. In the constricted pedicellarise, the loop is usually very well developed, 

 and a marked constriction separates the blade from the base of the valve; the 

 blade itself may be nearly circular or more or less elongated, but it usually 

 contains a very considerable calcareous mesh-work. 



The triphyllous pedicellarise seem to be constantly present, though on account 

 of their very small size, they are often difficult to find, especially when the other 

 sorts of pedicellarise are very abundant. Their heads are usually from .10 to .20 

 mm. long and are borne on stalks four or five times as long, with which they are 

 connected by a very extensile neck. The valves themselves show httle diversity 

 in form and scarcely any structural peculiarities. The distal half is commonly 

 wider than the base and is distinctly truncate, though in a few species it is 

 rounded. The breadth of the valve is usually about equal to the length and may 

 exceed it. In most cases the ending of the apophysis in the blade is quite indis- 

 tinct, but it may appear as a well-marked fork and rarely it gives rise to ridges 

 which run out onto the blade. 



The sphccridia show very little indication of speciahzation. They are present 

 in some numbers on the actinal part of each ambulacrum, but are not sunken 

 in any depressions, nor have they any unusual relation to the plates. They 

 occasionally occur on the buccal plates, and as Alortensen has shown, their 

 intergradation with miliary spines is sometimes quite evident. They are more 

 or less elongated and are usually smooth but may be quite rough, at least at the 

 tip. 



The calcareous spicules of the tube-feet, when fully developed, are of the 

 typical bihamate form, but they are often found with the ends blunt instead of 

 sharp. Mortensen ("Ingolf" Eeh., pt. 1, PI. 21, fig. 31) has given a good series 

 of figures showing the transition from simple granules to dumb-bell shaped 

 spicules and from the latter to complete bihamate rods. Sometimes the bi- 

 iiainale sj)icules are more or less branched or provided witli teeth at and near 

 tli(> lip. Spicules are sometimes abundant but are often uncommon and very 

 liani to fliid. and are not of the least value for systematic purposes in this family. 



Tin; (Jknkha and Species of Recent Echimd.e. 



There are rather more than fifty recent species, belonging in this family as 

 here hniitod; the fossil forms unfortunately must he left out of account. 

 The recent species form a homogeneous group ami it is difficult to 



