868


Journal of Paleontology 92(5):850–871


parallel to plate boundaries. Radial circlet ~67% of aboral cup height; radial plates five, A, B, D, and E radial plates simple, of approximately same size and shape; symmetrical about oral- aboral axis; C radial plate compound. A, C, and D radial plates supporting a free arm; B and E radial plates lacking a free arm, but each with recumbent ambulacrum. Sculpture of A, C, and D radial plates only coarse irregular nodes of similar size across plates (Fig. 9.6). Sculpture of B and E radial plates with coarse nodes marginally; large nodes toward plate center merging into coarse ridges that abut and are arranged perpendicular to recumbent ambulacra. C inferradial hexagonal, approximately as high as wide; superradial above to right; first anal plate above to left. Radial facets angustary, horseshoe-shaped, declivate; details of radial facet topography unknown. Posterior interray and tegmen plating unknown on present


material; surface presumably perpendicular to oral-aboral axis. Atomous free arms three (A, C, andDrays). Primibrachials


rectilinear uniserial, as high as wide, >3.0 times deeper than wide proximally, as deep as wide distally; lateral sides of brachials with coarse irregular nodes and ridges that define a groove admedially along side of each brachial (Fig. 9.6). On rays with free arms (A, C, and D), ambulacra extending distally along adoral side of arm, over distal tip of arm, proximally along aboral side of arm, onto radial plate surface, and typically onto surface of basal plate. In some instances, ambulacrum extending onto proximal surface. On rays without free arms, ambulacra typically extending proximally onto basal plates and can extend onto column (Fig. 9.6). Biserial cover plates covering ambulacra. Proximal columnals very thin, heteromorphic, holomeric,


convex latus; proximal columnals very low, becoming higher distally. Lumen morphology and columnal facet morphology unknown.


Materials.—As noted above, the original Wetherby (1880) specimens have not been located, but existing collections include those from Springer (1911): USNM S 2051 and 2051; from Sprinkle (1973): USNM S 5732−5734; and from Parsley (1981): USNM 245187 and 245189. New specimens include UMMP 74670.1–74670.5 and 74687.3.


Measurements.—UMMP 74670.1: CrH, 28.0*; CaW, 18.5*; CoH, 10.0*. UMMP 74670.2: CrH, 25.3; CaH, 13.2; CaW, 17.0*; CoH, 8.0*. UMMP 74670.3: CaW, 18.6; CoH, 10.0*. Brechin UMMP 74670.4: CrH, 27.6*; CaH, 16.2; CaW, 20.1*; CoH, 13.0*. UMMP 74670.5: CrH, 28.6;* CaH, 21.0; CaW, 20.7; CoH, 12.5*.


Remarks.—Two species of Hybocystites are recognized, H. problematicus (originally described from Kentucky, USA) and H. eldonensis (originally described from Ontario, Canada). The type specimen of H. eldonensis was “only about seven millimeters in vertical extent” (Parks, 1908, p. 234), and it was differentiated from the type species by having the two recum- bent ambulacra confined to the radial plates, and an ambulacral furrow that does not extend proximally to radial plates in arm- bearing rays. The Parks species also has much more subdued aboral plate sculpture than H. problematicus. Springer (1911) made a convincing argument that the differences between H. problematicus and H. eldonensis can be explained as


ontogenetic changes. Further, in both Kentucky and Kirkfield, Ontario, single clusters of Hybocystites specimens contained both H. problematicus and H. eldonensis forms. Springer sug- gested that these two taxa are conspecific. He stated, “In view of the fact that Dr. Parks was the first to investigate and describe the Canadian forms, I prefer to place the foregoing facts at his disposal and leave the decision of the question to him, hoping that he will publish his conclusion; but the weight of evidence seems to me to favour the latter view” (Springer, 1911, p. 21, 22). Parks did not address this question in subsequent publica- tions. Also, note that both the H. problematicus and H. eldonensis forms co-exist in the Brechin material. Parsley (1981) followed Springer’s conclusions and


designated Hybocystites eldonensis as a junior synonym of H. problematicus. Synonymy of these two species has not always been followed (e.g., Webster and Webster, 2013), but we agree with Springer (1911) and Parsley (1981) that the ‘weight of evidence’ supports the position that these two species are conspecific.


Acknowledgments


We thank the following for assistance with curated specimens: K. Hollis, National Museum of Natural History (NMNH), Smithsonian Institution, and B. Hunda,CMC. S. Rozhnov, G.D. Sevastopulo, and C. Sumrall improved an earlier version of this manuscript. SRC was supported by a Presidential Fellowship from Ohio State University and a Springer Fellowship from the NMNH. DFW was supported by a James R. Welch Scholarship from the Association of Applied Paleontological Sciences and a Peter Buck Fellowship from theNMNH. This research was also partially supported by the National Science Foundation (DEB 1036416).


Accessibility of supplemental data


Data available from the Dryad Digital Repository: https:// doi:10.5061/dryad.dm67f.


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