148 



THE CRINOIDEA 



l.a. Br 



absent, its place being occupied by anal tube ; the tube encroaches on 

 r. ant. arm, so that this too disappears ; r. post. Rs and r. ant. Rs fuse 

 to form a T-shaped piece supporting anal tube ; the T-piece atrophies, 

 and the tube then rests on the corresponding inferradials ; the simple RR 

 (I. post, and ant.) increase in size, forming the sides of the cup and bend- 

 ing round on the adcolumnal side where the anal tube is, as well as on 

 the acolumnal side, where they eventually meet between the two halves 

 of 1. ant. R ; the arm borne by 1. ant. R may fork once, but usually 



FIG. LX. 



Calceocrinits tucanus, from the anterior side. 

 }}, the base, hinged to left anterior radial, 

 which is out of sight, and flexibly jointed to 

 r.a.Ri, right anterior inferradial ; the super- 

 radials of this and of the corresponding radial 

 on the other side are hidden by the series of 

 main-axils, lAx to VAx, which support the 

 branches of the t\vo large side-arms; the 

 visible side-arm, a.Br, is the anterior, and 

 springs from a.R, anterior radial ; its diminished 

 branches are seen at y and S, its enlarged 

 branches at r ; the single arm along the upper 

 side is the left anterior(Z.a..Br). (After Bather, 

 1893.) Natural size. 



remains simple, owing to the extraordinary development of the arms 

 borne by the two large RR on either side of it ; the adcolumnal ramus 

 of each of those arms is reduced to a series of 3-8 axillaries (main-axils) 

 which lie side by side, curving round towards the anal tube ; each main- 

 axil gives off a branch which itself bears armlets, which in turn may 

 assume the regular nature of pinnules, and are rarely visible on the 

 exterior of the folded arms ; the acolumnal ramus dwindles in size and 

 becomes hidden by the other branches ; r. post. B atrophies, 1. post, and 

 1. ant. BB fuse, and the three plates thus left form a triangular base, 

 which is regularly hinged to the 1. ant. Ri, so that the crown could move 

 up and down in the sagittal plane of its bilateral symmetry ; on the other 

 hand, the columnals have each a slight curvature in this plane, and this 



JAx 



a. Br 



VAx 



FIG. LXI. 



Diagrams illustrating the structure of the posterior area in 1, Casta^rinus ; 2, Euchirocrinus ; 

 3, Calceocrinus ; 4, Halysiocrinus. 



kept the stem itself rigid. The various stages in the evolution are 

 marked by 4 genera (Fig. LXI.) : Castocrinus, Ringueb. (1889), Ordovician, 

 N. America; Euchirocrinus, Meek & Worth. (1873 ; synn. Cheirocrinus, 

 Hall non Eichw. ; Cremacrinus, Ulr. ; Proclivocrinus, Ringueb.), Ordovician 

 and Silurian, N. America; Calceocrinus, Hall (1852, em. Ringueb., 1889 ; 

 synn. Cheirocrinus, Salter, nom. nud. ; Pendulocrinus, Austin, MS. ; ? Delta- 

 crinus, Ulr.), Silurian and Devonian, N America and Europe ; Halysio- 

 crinus, Ulrich (1886, em. Bather, 1893), Carboniferous, N. America. For 

 history and morphology see Bather (1893). 



