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



of rare species more exact and definite information than has yet been 

 afforded. But little light is thrown upon the subject by a reference 

 to the characters which obtain in Brisinga, owing to the extremely 

 generalized character of that important form ; the adambulacral 

 spines are, according to the careful description of G. O. Sars, arranged 

 in the adult in three longitudinal rows, of which those of the outer- 

 most row are distinctly the longest ; " in very young specimens," 

 however, it is to be noted that the "two innermost are yet undeve- 

 loped, so that there is only a single longitudinal row of furrow- 

 spines along the middle of the adambulcral areas." 



Although there is this tendency to a disposition of the adambula- 

 cral spines in one or two rows, there are (a) cases, e. g. A. panopla, 

 Stuxberg, in which there are three rows, and (/3) specimens of the 

 common and widely-spread A. rubens and of other species which do 

 not always exhibit a constant regularity : the spines may be set more 

 or less distinctly along three planes ; but it is rarely that three spines 

 are found on one and the same plate ; and it is clear that there is a 

 tendency to the development of a single continuous series, arranged 

 as nearly as possible in a perfectly straight row. If, further, we con- 

 sider how extremely short is the longitudinal axis of the ossicles, and 

 the size of the spines that are placed on them, and that the packing 

 of the spines does nevertheless diverge but little into any thing what- 

 soever of a zigzag arrangement, we are, I think, justified for the 

 present in not enforcing any such further subdivision as would be 

 expressed by such terms as isacatithid and anisacanthid.^ 



There is a considerable body of fact which would justify such a 

 division ; but there is need of further study and fuller information to 

 enable us to speak definitely as to the permanency of any real anisa- 

 canthid arrangement ; the species in the following lists, which are 

 distinguished by an asterisk sign are those in which this irregularity 

 is most conspicuous. 



III. Primary Groups of the Species of Asterias. 

 A. HETERACTINIDA. 



I. POLYPLACIDA. 

 i. MONACANTHIDA. 



calamaria. tenuispina. 



ii. DlPLACANTHIDA. 



*acutispina. pohjplax. 



capensis. 



II. MONOPLACIDA. 

 i. MONACANTHIDA. 



gelatinosa. perrieri. 



gemmifer. rodoljphi. 



katherince. 



* These terms would, of course, express the difference between forms in which 

 the spines are the same in number on all the plates, and those in which the spines 

 are iiuequally distributed. 



