2Q2 SMITHSONIAN INSTITUTION 



CLEISTOCRINUS nov. gen. 



(jolaaTog, closed; xplror, lily) 



Plate XXXVIII, figs. 1-2 



Fig. 37. Cleistocrinus 



Ichthyocrinidae with rays in contact except at anal side. Crown expanding" 

 distally. Infrabasals entirely within the ring of basals, or wanting. Posterior 

 basal not differentiated. Radianal in form of radial under right posterior radial. 

 Anal x followed by others in series, usually not connecting with basal. Inter- 

 brachials wanting. Primibrachs two. Arms unknown. Column large, cover- 

 ing basals. 



Genotype. Calpiocrinus humilis Angelin. 



Distribution. Silurian, Wenlock Group ; only known from Gotland. 



I have proposed this genus to receive a form which, although represented by rather 

 imperfect material, is nevertheless of a very definite type, and occupies a place in the present 

 family which cannot be confounded with any other. It is founded upon the Silurian species 

 described by Angelin as Calpiocrinus humilis. His figures show very little of the essential 

 structures which make the form important, and which are now clearly brought out by new 

 figures made after further preparation of the specimens, especially by removal of the stem 

 ossicles filling the basal cavity in one of them, thus enabling us to see the true arrangement 

 of plates which was before obscured. 



This form stands close to Ichthyocrinus, but with the posterior rays partly separated by 

 anal plates ; as in that genus the posterior basal is not differentiated, but a strong series of 

 anal plates is interposed above the level of the radials, one of which and the radianal meet 

 below, entirely separating the anal series from the basal circlet. This is a very unusual 

 structure in crinoids, but is found also in one species of Calpiocrinus; therefore I have not 

 made it an absolute generic character, and a species with anal plate coming down to the basal 

 would be admitted. The extreme diminution of the base, however, leads me to think that 

 the non-differentiation of the posterior basal is probably a well correlated character. There 

 is in one specimen a slight tendency to development of interbrachials, but it is evidently only 

 sporadic, as is seen occasionally in Metichthyocrinns. A rather clear line of succession leads 

 from this Silurian type through Synaptocrinus in the Devonian with no radianal, greatly 

 elongated posterior basal and no connecting" anal structures, to Euryocrinus in the Car- 

 boniferous, which adds the anal series and interbrachials together with another primibrach. 



The atrophy of infrabasals so frequent in this family has proceeded to the extreme of 

 resorption in this genus. In the two specimens known not the slightest trace of these plates 

 remains within the ring of basals ; not only so, but the basals themselves seem to be on the 

 verge of suppression. In one specimen they are much encroached upon by the axial canal, 



