ICHTHYOCRINIDAE 293 



and in the other nearly half of them have disappeared (PI. XXXVIII, figs, id, 2b). The 

 basal structures are deeply buried under the large proximal columnal to an extent only 

 paralleled in some species of Dactylocrinus; here the basals, radials, and a part of the first 

 primibrachs are involved in the column facet. None of the specimens are sufficiently pre- 

 served to show' the arm structure, but- it was probably not much different from that of 

 Ichthyocrinus. Only the one species is known. 



Cleistocrinus humilis (Angelin) 

 Plate XXXVIII, figs. 1-2 



Calpiocrinus humilis Angelin, Icon. Crin. Sueciae, 1878, p. 12, pi. 23, figs. 28, 28a, b, c, pi. 26, fig. 17. — 

 Wachsmuth and Springer, Revision Palaeocrinoidea, pt. 1, 1879, p. 39. 



Type of the genus. 



A small species. Calyx rapidly spreading; height to width at top of I Ax, 

 1 to 3.6; spread from base, i to 2.2. Side outline a low curve; cross-section cir- 

 cular; base very large, deeply concave, involving all plates below IBr. Surface 

 smooth. Full dimensions of crown unknown; the principal specimen is 18 mm. 

 wide at top of IBr; basal concavity 8 mm. measured from perimeter of column; 

 actual diameter of basal ring, 2.5 mm. IBB resorbed, and sometimes in part 

 also the basals which are very small. RA as large as some of the radials, which 

 are somewhat irregular in size. Anal x a long plate, sometimes rising as high 

 as the axillary IBr, and followed by one or more others in series ; it is angular 

 or truncate below, supported by 1. post. R and RA. RR small, much wider above 

 than below; IBr continue to increase in width in about same degree. Superior 

 parts unknown. Column very large, composed next to the calyx of thin ossicles. 



The species is only known from two specimens, neither of which has the rays preserved 

 above the primibrachs, and of these the right posterior axillary is wanting in both. Figures 

 2a, b show that the second primibrach in that ray alone is not axillary ; so there must be a 

 third above it, and the small unsymmetric plate at the base of the ray in both specimens must 

 be a radianal. 



Types. Riks Museum, Stockholm, Sweden. 



Horizon and locality. Silurian, Wenlock Group, horizon /; Gotland, Sweden. 



