FOSSIL CRINOIDS. 135 



but having a fairly well defined and nearly constant transverse ridge. This 

 is distinctly shown in Miller's figures of P. crassus, the type species (Nat. Hist. 

 Crin., opp. p. 68, figs. lA, and 2H to 5H). It also appears clearly in the follow- 

 ing illustrations of other early species: — P. plicatus Austin, Mon., PI. IX, fig. 

 4c; id., de Koninck and Lehon, Crin. Carb. Belg., PI. I, fig. 11; P. radialus, 

 Ibid., PI. I, fig. 12. De Koninck's figure 10, of P. crassus, shows a very different 

 structure, and the specimen may not belong to the genus. 



The same type of articulation obtains in the three species herein illustrated. 

 In all are seen the same deep plication of the calyx plates, which is not a mere 

 surface character, but is due to a folding in the substance of the plates. This, 

 with modification from folds to pits, is a usual, and probably constant character 

 in the genus. 



As already stated, one objection to the use of the family name, Poterio- 

 crinidae, has been that the genus Poteriocrinus is not typical of its family, in not 

 possessing in full the second of the characters above stated as distinguishing 

 it from the largest other Inadunate family, the Cyathocrinidae, viz, a wide, 

 straight radial facet. In discussing this distinction Wachsmuth and Springer 

 (Rev. Pal., Ill, 189-190) explained that the radials of the Cyathocrinidae have 

 horseshoe-like facets for the brachials; that in the Poteriocrinidae they are 

 more or less truncate along the upper margin, and united with the brachials by a 

 transverse ridge, frequently extending their entire width, accompanied by more 

 or less conspicuous fossae for muscles and ligament; the same mode of articula- 

 tion existing also upon the axillary brachials; that in the rays of the Cyatho- 

 crinidae there are, so far as observed, no muscular fossae, neither between radials 

 and brachials, nor upon the axillaries; the apposed faces of all their joints fit 

 closely together, the distal end being slightly concave, the proximal to the same 

 extent convex. 



In order to have a basis for some consideration of the possible significance 

 of these two seemingly wide differences in the mode of articulation of the arms, 

 it may be well to give a description of the radial facet as it exists in thfe living 

 Crinoids. For an accurate statement of this I am indebted to Mr. Austin H. 

 Clark, who has made these articulations the subject of special studies, based 

 both upon his own observations and the work of previous authors : — 



Primarily the articular facet in all groups of Recent Crinoids is composed 

 of (1) the dorsal ligament fossa; (2) the transverse ridge; (3) the interarticular 

 ligament fossae; and (4) the paired muscular fossae, these last separated by 

 either an intermuscular ridge or an intermuscular furrow, extending usually 



