44 UINTACRIXUS: ITS STRUCTURE AND RELATIONS. 



Among the Flexibilia * (= Articulata, sensii W. & Sp.) there are a number 

 of interesting cases. In the Palaeozoic genus Taxocrinus the ventral surface 

 is covered by an integument of very small, irregular, calcareous pieces, on 

 the surface of which five well-defined ambulacral grooves, with covering 

 plates and side-pieces, converge in the centre and pass down into an open 

 mouth between five large and conspicuous oral plates.f 



The mouth is central, and the anal tube originates laterally, well down 

 between the two posterior rays. It is probable that the tegmen of the 

 Flexibilia generally was of a similar nature. 



In the Jurassic genus Apiocrinus, according to Loriol, | who found it in 

 a specimen of ^. roiss//amis, the ventral disk has the form of a conical sac, 

 covered with iri-egular, thin plates, rising from the heavy interbrachials. 

 No ambulacral grooves could be traced, nor could the position of the mouth 

 be determined. Mr. A. Agassiz § is inclined to consider the conical sac of 

 de Loriol's specimen the anal proboscis, — a surmise which I think highly 

 probable, in view of the similar form and appearance of the anal protuber- 

 ance in Uintacrinus. But whether it was central or not we cannot learn 

 from the specimen. 



Calamocnnus,\\ the recent representative of the Apiocrinidae, has a teg- 

 men composed of small plates suturally connected ; the mouth is central, 

 ambulacral grooves roofed by movable covering plates, and the anus is at 

 the end of an excentric tube. 



In Rhizocnniis, the living representative of the Bourgueticrinidfe, the 

 mouth is central, protected by five small oral plates, which occupy the cen- 

 tral ends of the interpalmar (interambulacral) areas ; the remainder of these 

 areas is not plated ; the ambulacra have covering plates, but no side plates. 



* So muoli confusion has arisen from the use of J. S. Miller's name " Artieulata " in different senses, 

 that I do not consider it advisable to retain it in the sense used by Wachsmuth and Springer in our Revision 

 and Monograph. In the latter work (p. 153) we intimated our misgivings in regard to it, and suggested an 

 alternative name " Articulosa," which proved to be preoccupied by Jaekel for a group a little differently 

 limited. Von Zittel has proposed " Flexibilia" for the same general group as ours. It is appropriate, free 

 from confusion, and in all resi)ects preferable. I propose to adopt it in a future work embracing this group, 

 and will therefore use " Flexibilia " in this paper, with the understanding that it is the equivalent of Articu- 

 lata as defined by us in the works above mentioned. 



t Wachsmuth and Springer. Discovery of the Ventral Structure of Taxocrinm. Proc. Acad. Nat. 

 Sci. Phil., 18S8, p. 345, PI. XVIII., Fig. \e. 



% Note sur quelques Echinodermes fossiles des environs de la Rochelle. Acad, de la Roclielle, Societe 

 des Sciences Naturelles de la Chareute Inferieure. Annales de 1886, Tom. XXIII. , 1887, p- 313. 



§ Calamocrims Diomeda, p. 42. Mem. Mus. Comp. Zool. at Harvard College, Vol. XVII., No. 2, 

 1892, p. 42. 



II Ibid. 



