NO. 1846. ON CERTAIN ELEUTHEROZOIG PELMATOZOA— KIRK. 81 



this genus, for many of the species referred to Millericrinus have such 

 a type of articulation. A careful examination of these forms may 

 prove that they are after all referable to the Pentacrinidae rather than 

 to the Apiocrinidae. The high degree of differentiation of the proxi- 

 male in certain species of Millericrinus may indicate a tendency within 

 the genus toward the formation of Comatula-like types by a dis- 

 ruption of the column immediately distad to this ossicle, after the 

 manner of Thiolliericrinus. Such a type of structure has already 

 been noted in the case of M. lyrattii, although cirri were not 

 acquired by this form. 



Relationships of the Comatulse. — Having briefly shown some of the 

 reasons for assuming that the Comatulae represent terminal members 

 of a number of genetic hnes, it may be well, broadly, to indicate the 

 relationship of these forms to the Crinoidea at large. Bather and 

 other authors have referred the Comatulas, together with a number of 

 other post-Paleozoic Crinoidea, to the Flexibilia. For the reception 

 of this assemblage Bather has erected the Grade Pinnata. I beheve 

 that the Pinnata should be transferred bodily to the Order Inadu- 

 nata. The presence of a persistent proximale, if there be such a 

 columnal, has certainly not been estabUshed in the case of these 

 forms, as has elsewhere been pointed out. On the other hand, the 

 structural affinities of these organisms seem to he far more with the 

 Inadunata. Among all the kno\\'n post-Paleozoic Crinoidea there is 

 an essential unity of structure that points strongly to a not widely 

 diverse origin. The ancestors of these Crinoidea may not, I think, 

 be found in any kno\\Ti Paleozoic genera, but must be sought in early 

 Mesozoic or late Paleozoic forms of minute dimensions. 



TYPE 2. 



The genera referable to this group probably constitute the only 

 Crinoidea that maintain a truly eleutherozoic existence through 

 their owm exertions as freely swimming organisms. With them 

 detachment from the column comes at a very early stage and results, 

 as I hold, in the complete loss of that organ. The space inclosed by 

 the proximal circlet of plates — either basals or infrabasals — through 

 wliich the axial organs passed in the stalked ancestral forms is closed 

 by a pentagonal plate, to which Bather has apphed the name "cen- 

 trale." Concerning the nature and origin of this plate it will be 

 necessary to go into considerable detail. One of the most curious 

 features about these crinoids is their apparent lack of antecedents. 

 Wlien first seen they are full-fledged pelagic types of a most remark- 

 able degree of speciaUzation, and it is a matter of exceeding difficulty 

 even approximately to predicate the nature of their progenitors 

 except in a very broad way. 



94428°— Proc.N .M. vol.41— 11 6 



