82 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



The three genera constituting this group have been described and 

 illustrated so adequately by various authors that it will scarcely be 

 necessary to give any description of the forms other than may be 

 required in a consideration of the broader aspects of certain struc- 

 tural features here discussed. Of prime importance in a considera- 

 tion of these types is the question of the origin and homologies of 

 the central apical plate, or centrale, wliich has been the subject of a 

 considerable amount of speculation. This topic bears largely not 

 only on the structure and relationships of these three genera, but also 

 on many broader morphologic and taxonomic questions. 



Bather on the "centrale.^' — Bather (1896, p. 979), in speaking of the 

 centrale of TJintacrinus, says: 



The centrale is pentagonal, but in specimens examined not quite regular * * *. 

 It is perfectly smooth, showing no signs either of a stem-attachment or of partition into 

 more than one original element. Its homologies are therefore doubtful, as its struc- 

 ture and position permit it to represent either a relic of a stem or a fused infrabasal 

 circlet, or even, as some would have it, an additional element altogether, to which the 

 name " dorsocentral " might strictly be applicable. I have recently given reasons for 

 rejecting the term and the idea "dorsocentral." Which of the other alternatives be 

 correct is to be decided, if at all, by reference to the affinities and origin of the genus, 

 as to which we are at this stage of the inquiry quite in the dark. 



Referring to the group comprising Saccocoma, Marsupites, and 

 TJintacrinus, he says that the central apical plate may originate in 

 three ways (1896, p. 997) : 



One may say, if one chooses, that in Saccocoma this represents the fused basals and 

 in Uintacrinus the fused infrabasals; but in Marsupites it must be something else. 

 Or one may say that in each case it is the same element, be it the proximal stem- 

 ossicle (which some erroneously call "centrodorsal") or the distal stem-ossicle (which 

 some, seeking an homology, have called "dorso-central"), or perhaps a new plate 

 altogether, a simple supplementary plate developed to fill up the gap left by the 

 disappearance of the stem. One might argue forever; there is no evidence. The one 

 obvious fact is that such a central plate is found in three different forms, all of which 

 were free-swimming, and unlike all other crinoids in showing absolutely no trace of 

 a stem. It is therefore not safe to ascribe to the central plate any morphological 

 significance or to give it any name other than "centrale." 



In 1900 (1900, p. 135) Bather modified his statement in regard to 

 the nature 'of this plate in Uintacrinus. 



* * * in Uintacrinus and Marsupites it represents neither basals nor infra- 

 basals, but may be the proximale, or the supposed distal columnar plate ("dorso- 

 central"), or a new supplementary plate. 



Springer on the "centrale.^' — Springer (1901, p. 22), in discussing 

 the centrale of Uintacrinus, quotes the statements made by Bather 

 in his paper on Uintacrinus, and seeks (1901, p. 28) to arrive at a 

 more definite conclusion in regard to the origin and homologies of the 

 plate. It will be noticed that he deals only with the suggestions of 

 Bather as presented in his Uintacrinus paper and not as subsequently 

 modified in Lankester's Zoology. He says in part (p. 28) : 



