64 THE DEVONIAN OF MISSOURI. 



slightly wavy. Plates below IIEta concave, with elevated rims and centers; sutures 

 deeply incised between ridges. Tegmen papilose. 



Basals 3; funnel deep, rim broad and rounded; axial canal 1/4 diameter of base, 

 quinquelobate; proximal columnals circular, funnel-shaped. 



Primaxials larger than radials. IIBr2 variable, pentagonal to cuneiform, IIBrj 

 et seq., biserially arranged. Arms bifurcating once; all IIBr pinnulate and incorporated 

 to IIBr 6 . Free portion of nP lost, nP canals closed. 



Interbrachials 2, perhaps 3, in larger specimens, not meeting ilAmb. 



Tegmen very low, flattened; all plates conforming in outline to generic plan; sur- 

 face of plates papilose. Anal opening slightly excentric, small. 



Column unknown. 



Stereocrinus vandiveri differs from the other species herein cited in the concavity 

 of all plates below IIBr2, flatness of the tegmen and bulging of the sides below the arm 

 bases. 



Occurrence— This is a common species in the Mineola limestone of Ralls County 

 and is probably abundant in the Mineola of Montgomery and Warren Counties, al- 

 though, as all specimens from there are exfoliated, it is impossible to identify them. 



The species is named for Mr. V. W. Vandiver, a student in the University of Mis- 

 souri, who collected the best specimen. 



A small specimen is probably an immature S. vandiveri as it has the tendency to 

 concavity in the plate surfaces, the same general style of ornamentation, and the deep 

 marginal incisions between the ridges. In this specimen several interesting points in 

 the development of incorporated pinnules are demonstrated. 



The pinnules of IIBri and IIBr 2 , incorporated as described in the generic plan, are 

 elevated slightly above the general surface of the tegmen, and several of the better 

 preserved ones bear a small pinnule facet, showing that the free portion of the pinnule 

 was very slender. The neuro-ambulacral canals, which are usually in contact with the 

 facet, are here slightly above it. This fact coupled with the small size of the facet ap- 

 parently means that atrophy of the pinnule had already begun. A larger specimen 

 shows complete absence of the canals and closer conformity of the incorporated with 

 the body curvature. 



Family Batocrinidae 



Genus Megistocrinus Owen and Shumard 



The North American species of Megistocrinus form two reasonably distinct groups 

 well separated stratigraphically but not differing to any great extent morphologically. 

 The first group, the Devonian species, has in general 16 arm openings in the calyx, a 

 strong anal tube, and few if any biserially arranged brachials incorporated. The second 

 group, the Mississippian species, has 20 arm openings in the calyx, a weak anal tube, 

 and many biserial brachials incorporated. This difference is evidently due to increasing 

 enlargement of the calyx which incorporates both tube and brachials in much the same 

 manner as incorporation takes place in the ontogenetic development of many recent 

 crinoids, and is not of sufficient importance to warrant separating the Devonian from the 

 Mississippian forms. 



Megistocrinus (Devonian) 



Calyx large, generally wider than high; convex, flattened, depressed, or excavated 

 aborally; convex to conical, or sometimes flattened orally. Plate surfaces generally 

 smooth, either conforming to the cup curvature, convex to nodose, or ornamented with 

 ridges radiating to the plate angles. 



Base hexagonal, with shallow depression for columnals; axial canal quinquelobate, 

 1/4 to 1/3 the diameter of the base. BB 3, hexagonal, sometimes completely ankylosed. 



