NO. 1846. ON CERTAIN ELEUTHEROZOIC PELMATOZOA—KIRK. 71 



this structure has the importance assigned to it, but that it is a 

 character independently acquired in several lines of evolution. 



It is evident that the forms with a proximale have been derived 

 from those having normal stem structure where new colunmals are 

 formed immediately beneath the proximal circlet of thecal plates. 

 If this character were acquired in a single genetic Une it would 

 doubtless be of considerable value, although it would be difficult at 

 best to distinguish between the primitive forms having such a col- 

 umnal and those from which they were derived. We are dealing, 

 however, I think, with a type of structure which may be shown to 

 have been independently acquired by numerous offshoots from the 

 Inadunata. 



It is a question whether there consistently be such a proximale as 

 a matter of fact. We may certainly postulate its existence in many 

 adult crinoids, but it appears to me highly probable that in the ontog- 

 eny of many such forms we might well find fresh columnals forming 

 immediately beneath the theca. In a number of forms referred to 

 the Flexibilia in wdiich the so-called proximale is notable because of 

 its high degree of differentiation it is probable that we may frequently 

 have the formation of new columnals proximad to this ossicle. Such, 

 for instance, is apparently the case in the genus Millericrinus. 



The appearance of an apparently persistent proximal columnal in 

 Millericrinus is of considerable interest inasmuch as it seems to have 

 been but imperfectly acquired within the genus. In such a form, for 

 example, as M. gracilis the proximal columnal may be seen to be but 

 slightly differentiated. In 21. prattii as described above (p. 51) the 

 proximal columnal, although apparently well formed, seems at times 

 to be superseded by the formation of a new plate between it and the 

 theca. In Millericrinus recuhariensis as described and figured by 

 Bather (1897), a species that Mr. A. H. Clark refers to Bathycrinus, 

 the proximal columnal is said "to be incomplete; it only occupies a 

 portion of the basal surface of the cup, and some of the basals rest, 

 in whole or part, on the columnal next following." It ■will be 

 unnecessary to enter further into a discussion of tliis primitive Milleri- 

 crinus, but I think it is evident that we ^lave here good evidence of the 

 formation of a new columnal between the former proximal columnal 

 and the theca. Again, wdtliin the genus we have cases in which the 

 proximal columnal appears quite as highly differentiated as in any 

 form to which a proximale has been ascribed. 



In certain species, as in M. charpyi (PI. 7, fig. 1), and less notably 

 perhaps in M. heaumonti (PI. 7, fig. 2), the proximal columnal is of 

 great height and is considerably differentiated from the remainder of 

 the colunmals. In such cases one would not be surprised to find 

 detachment becoming effective immediately beneath tliis columnal, 

 and the ciinoid assuming; an eleutherozoic existence. We know that 



