LECANOCRINIDAE 203 



Nipterocrinus arboreus Worth en 

 Plate XV, figs, p-io 



Nipterocrinus arboreus Worthen, Geol. Surv. Illinois, V, 1873, p. 436, pi. 4, fig. 8. — Wachsmuth and 

 Springer, Revision Palaeocrinoidea, pt. 1, 1879, P- 56. 



This Lower Burlington representative of the genus is similar to the type 

 species, so far as can be ascertained from the specimens, in everything except 

 the number of brachials beyond the first, the IIBr being 4 or 5, and IIIBr 6 or 7. 

 This character is constant in two very well-marked specimens ; and it makes the 

 rays appear more robust, the brachials being relatively shorter and wider. The 

 stem is unknown beyond a few proximal joints. There was nothing in 

 Worthen's description to differentiate this species from the type, and the above 

 grounds, not mentioned by him, are the only ones upon which their separation 

 may be maintained. 



Types. Worthen's original is in the University of Illinois ; that of figure 10 in the 

 author's collection. 



Horizon and locality. Lower Carboniferous, Lower Burlington limestone ; Burlington, 

 Iowa. 



Nipterocrinus wachsmuthi Meek and Worthen 

 Plate XV, figs. 1-8 



Nipterocrinus wachsmuthi Meek and Worthen, Proc. Acad. Nat. Sci., Philadelphia, 1868, p. 342; Geol. 

 Surv. Illinois, V, 1873, p. 435, pi. 2, fig. 4. — Wachsmuth and Springer, Revision Palaeocrinoidea, 

 pt. 1, 1879, p. 56. — Springer, Jour. Geology, XIV, 1906, p. 522, pi. 6, fig. 18. 



Type of the genus. 



A medium-sized species. Crown elongate. Calyx low, hemispheric, evenly 

 curved; height to width at top of RR, about I to 1.55; spread of calyx from 

 column facet, 1 to 3 ; side outline convex; cross-section circular. Surface smooth 

 or finely granular, without ridges or angularities. Crown of large specimen, 

 45 mm. high by 22 mm. wide. 



IBB low and flat, barely visible in side view ; undivided. BB narrow, nearly 

 as high as RR, but not appearing so in a side view because foreshortened. RR 

 by far the largest plates of the calyx, and much wider than high ; facets narrow, 

 about half the width of R, deeply curved, leaving rounded shoulders at either 

 side for attachment of perisome. IBr 3 01-4; IIBr 8 or 9; IIIBr 10 to 15 or 

 more, with another bifurcation only on the inner ramus of the dichotom. Arms 

 rounded and deep, with strongly curved sinuous sutures. Column very long, 

 large in proportion to the crown, not expanding - proximally, and with scarcely 

 any taper until near the end where a few radicular cirri or branches appear; 

 proximal columnals extremely thin, equal, with straight sides; about one-twelfth 

 as long as wide, gradually increasing in length without any alternation, until 

 the distal columnals become longer than wide. 



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