68 AKT. 1. — I. IJIilA : HEXACTINELLIDA, III. 



lip of 4-8, radially arranged, recurved teeth, whieli give a 

 w^atchglass-like or hemispherical oultine to the disc. — The form 

 in question occurs sparsely in the small specimens of PL V., fig. 

 9. It is common in the nut-sized specimen of PI. V., fig. 8, 

 while in still larger specimens it is either scarce again or is not 

 found at all. 



Form c is the most characteristic and the most constant of 

 the discohexasters in specimens of the species that have attained 

 a growth beyond the nut-size. A good idea of its appearance 

 may be obtained from PL IV., fig. 9. It is of a very large size, 

 though subject to a considerable variation in this respect like the 

 other forms of discohexasters. It commonly measures 240-340/^ 

 in diameter ; in the larger specimens of the species it may 

 sometimes be even so large as to measure 400 /^ in diameter. 

 The short principals are usually, though not always, so thickened 

 that each presents a rounded knob-like appearance. The long and 

 slender terminals, 2-4 (usually 3) in number to each principal, 

 are strongly divergent and not always straight in their course. 

 They are smooth and perceptibly thickened towards both ends 

 but somewhat more towards the outer end, whicli bears a reverted 

 umbel of 4-G, long, anchor-arm-like prongs. This terminal umbel 

 gives to the hexaster a very striking appearance. The cupola of 

 the umbel is rounded. The sharply pointed prongs may be 30/^ 

 long ; unlike those in a codouhexaster, they are often more or 

 less bent in an irregular way and proceed divergingly backwards 

 from the cupola, so that the umbel assumes the form of a bell 

 with a flarins; rim. — The above discohexaster-form undoubtedlv 

 represents the most advanced stage of development undergone by 

 the discohexasters of the species. It is still undeveloped in the 

 three smallest specimens depicted in PL V., fig. 1). With tolerable 



