CLYPEASTIilXA. • 17 



Altlitnigli aphaeridia occur in all known genera of C'lypeastrina, their small 

 size and more or less complete concealment within the calcareous matter of the 

 test itself makes them of little use for systematic purposes. It may be mentioned 

 in passing that the presence of depressions or even deep cavities within which 

 the sphaeridia occur is characteristic of the Arbaciidae alone among the fami- 

 nes of regular Echini. 



The calcareous particles in the tube-feet of the clypeastroids show the same 

 gradual decrease in size and importance that the pedicellariae reveal. This is 

 very probably associated with the change of function of the pedicels from loco- 

 motor to respiratory. In the less specialized genera, there is a well-developed 

 calcareous plate in the disk of each pedicel; this plate is perforated by a large 

 central ojiening and many small ones nearer the margin; it is more or less well 

 provided with projections on the outer margin. Besides this plate a few cal- 

 careous rods may be present in the wall of the foot but they are insignificant 

 and commonly wanting. In the Laganidae the terminal plate is less well devel- 

 oped than in Clypeaster and in most cases seems to be wanting as it is also in the 

 Fibulariidae. In the Arachnoididae, it is reduced to a very slender ring surround- 

 ing the tip of the pedicel. In the Scutellidae, it is entirely wanting but in several 

 genera is replaced by two rods, lying side by side at the center of the disk; 

 these rods are somewhat bent or cur\'ed, so that they are further apart at the 

 middle than at either end, and on the outer side is a projecting tooth or several 

 knobs. 



The Families of Clypeastrina. 



In the Revision of the Echini, Mr. Agassiz (1873, Rev. Ech., pt. 3, p. 505, 524) 

 recognizes two families of clypeastroids, the Euclypeastridae and the Scutellidae. 

 In the former, he grouped the genera into three subfamilies, Fibularina, Echi- 

 nanthidae, and Laganidae ; Arachnoides he placed in the Scutellidae, expressing 

 himself (p. 529) as strongly opposed to the removal of the genus from that family. 

 Duncan (1889, Journ. Linn. Soc. Zool., 23, p. 143) adopts essentially the same 

 classification but he raises each of the three subfamilies to full family rank, 

 calls the Echinanthinae by the more correct name Clypeastridae and recognizes 

 the peculiarities of Arachnoides by making it the only genus of a subfamily 

 "Arachninae" under ScutelHdae. He omits any subfamily name for the other 

 genera of the family, but no doubt he intended to use "Scutellinae." Gregory 

 (1900, Lankester's Treatise on Zoology, 3, p. 316) adopts the same four families 

 but removes Arachnoides from the Scutellidae altogether and makes it a separate 



