288 Field Columbian Museum — Geology, Vol. II. 



Achradocrinus patulus sp. nov. Plate LXXXV, Figures 1-4. 



Dorsal cup depressed, obconical, truncated at the base, somewhat 

 inflated on the posterior side so that the symmetry is bilateral. Sur- 

 face of the plates granulose and moderately convex, forming shallow 

 grooves in which the sutures are situated. 



Infrabasals small, forming a disc which barely extends beyond the 

 circumference of the column. The disc is provided with a central, 

 circular canal, and the plates appear to be fused as the sutures are 

 not visible. Basals five, four equal in size and shape, angular above, 

 the posterior larger and truncated to support the anal plate. Ra- 

 dials five, three shield-shaped and equal, the two posterior radials 

 deeply excavated for the reception of the anal plate which they en- 

 close on three sides by meeting above. The distal edges of the radials 

 are sharply inflected towards the center of the vault. A prominent, 

 semicircular, articular facet, directed outward, occupies more than 

 half the width of the radials. There is an axial canal situated at 

 about the center of the facet and a wide, shallow ventral groove ex- 

 tends from the facet across the inflected edge of the radials. The 

 anal plate is small, subquadrangular, its lateral edges arched and 

 its distal edge excavated for the anal opening. It rests upon the 

 posterior basal and between the posterior radials. The anal opening 

 is situated in line with the arm facets directly above the anal plate 

 and below the lateral extension of the adjoining radials. The opening 

 is directed horizontally and is surrounded by a number of small plates. 

 Arms unknown, but judging from the articular facets they appear to 

 be widely divergent or possibly recumbent. Tegmen not preserved. 

 Column circular with central canal. 



The species here described is referred to the above genus, although 

 differences from the genotype occur. These differences, however, 

 are not considered to be of generic importance. Schultze's original 

 figures are reproduced for comparison, PI. LXXXV, Figs. 5-8. 



With A. ventrosus, A. patulus compares as follows: In form and 

 arrangement of the plates the two species are similar, likewise in the 

 character of the stem and the articular facets with their ventral grooves 

 and axial canals. In A. ventrosus, however, the five infrabasals are 

 distinct and form a shallow cup visible in a side view of the calyx, but 

 in A. patulus the infrabasals appear to be fused into a disc and incon- 

 spicuous. The type specimen of A. patulus is, however, silicified and 

 it is quite probable that the basal sutures have been obliterated in the 

 process of silicification. The anal opening in both species is situated 

 between the distal edge of the anal plate and the lateral extensions of 

 the posterior radials; but in the genotype the opening is directed 



