748 THE CKINOIDEA CAMEEATA OF NOETH AlIEKICA. 



Remarks. — The form under consideration was defined in 1883 by 

 "Williams as Arthroacantha, but the name was afterwards changed by Hinde 

 to Hystricrinus, because it was, as he maintained, incorrectly formed, and 

 a name too similar to Arthracanilius, previously employed by Schmarda 

 for a genus of Rotatoria. We have formerly in Part TIT. of the Eevision 

 expressed the opinion that according to the rules of nomenclature Williams' 

 name would have to be retained, as it was sufficiently distinct from the 

 other, even if changed to Arthracantha. The same view of the case was 

 taken by Professor Whiteaves and Mr. S. A. Miller, both accepting Williams' 

 name. If the question were to be decided by some authoritative body of 

 naturalists, we should vote in favor of suppressing Arthroacantha and legi- 

 timizing Hinde's name, on the ground that no author should be permitted, 

 at this day, to establish a genus of Crinoids under any name which does not 

 end with the recognized termination — " criniis." As it is, we have con- 

 cluded, though with much reluctance, to retain the name proposed by 

 Williams, but writing it Arthracantha. 



Arthracantha is closely allied to Hexacrinus, from which it differs in having 

 biserial arms, and movable spines upon the calyx and arms. The spines 

 were probably attached to the plates by elastic ligaments, so as to yield 

 when accidentally brought in contact with other objects ; but we doubt if 

 they represent either functionally or structurally the spines of the Echini. 

 In this we differ from Williams, who thought that this structure establishes 

 a relationship between Crinoids and Perischoechinoidea. He compares them 

 with the spine-bearing plates of Lepidocentrus eifeliamis Miiller, and is led to 

 believe that these were probably plates in the " vault " of a true Crinoid 

 like Arthracantha. We can see nothing to support this view ; the Eifel 

 species is undoubtedly an Echinoid, and the plates of the two forms have 

 a superficial resemblance, but are not homologous. The movable spines of 

 Arthracantha, in our opinion, represent the sharp point of an ordinary spini- 

 ferous Crinoid plate, united with the basal portion b_y ligament*, and as 

 such are of but little importance in classification. We therefore consider 

 the mobility of the spines of only generic importance, differing therein 

 from Williams and Hinde, who were inclined to make Arthracantha the 

 tj'pe of a distinct family. 



