506 



PROF. F. J. BELL ON THE GENUS ASTERTAS. [May 3, 



e=echinoplacicl ; 

 a'=autacanthid ; 

 r=rarispinos8e ; 



a=anechinoplacicl ; 

 / = typacanthid ; 

 r'=retusispinos8e ; 



s = simplices; 

 c=acutispinos8e. 



Thus, the formula for A. rubens is 2 ats ; for A. rubens is dipla- 

 canthid, anediinoplacidj typacanthid, with simple dorsal spines. 



To distinguish between the Pentactinida and the Heteractinida 

 I propose to place the formula for the latter under the mathema- 

 tical sign of a square root ; thus sj \ p \s sufficient to distinguish 

 A. calamaria as a monacanthid polyplacid heteractinid form. If 

 we know, as we do in this case, further details, we may write the for- 

 mula sj 1 paa ; or, in other words, in addition, A. calamaria 

 has no spines round its madreporic plate, and the dorsal spines are 

 placed on special plates. 



To take two other examples : — 



A. epichlora = 2 atr, or is diplacanthid, anechinoplacid, typacan- 

 thid, and retusispinose (or has the spines blunted and short); the 

 absence of the root sign shows it to be pentactiuid. 



For A, gelatinosa we have >^ \ m ; ox it \s monacanthid and 

 monoplacid, while the root-sign shows it to be heteractinid ; as, also, 

 it is anechinoplacid and typacanthid, we may write its full formula 



