436 SMITHSONIAN INSTITUTION 



Unychocrinus distensus Worthen 

 Plate LXXIII, figs. 5-8 



Onychocrinus distensus Worthen, Bull. I, Illinois St. Mus. Nat. Hist., 1882, p. 31 ; Geol. Surv. Illinois, 

 VII, 1883, p. 307, pi. 29, fig. 5. — Wachsmuth and Springer, Revision Palaeocrinoidea, pt. 3, 1886, 

 P- I4S- 



A medium-sized species, also intermediate between the types of 0. exsculp- 

 tus and 0. ramulosus ; in general appearance similar to the former, but differ- 

 ing in the small number of iBr and longer rami. IBr variable, 3 or 4. iBr 

 consisting of one large plate in the first axil followed by perisome, but none so 

 far as observed in the second. Anal tube rather small. Dimensions from 

 average of 8 mature specimens: Length of ray to fork, 18 mm.; of ramus 

 above fork, 40 mm. ; diameter of calyx at IBr 3 , 24 mm. ; of base, 7 mm. Clus- 

 ters of ramules, 9 to 15. 



This species as described by Worthen was founded upon a specimen from Monroe 

 County, Illinois, in strata of the lower Kaskaskia now called the Renault formation. It 

 formed a part of an interesting crinoidal fauna occurring at a locality on Prairie du Long 

 Creek, which I have since carefully investigated. 1 The horizon has now been determined by 

 the geologists of the U. S. Geological Survey to be equivalent to certain beds at Huntsville, 

 Alabama, formerly considered to be St. Louis. The type specimen is imperfect, and for 

 better illustration of the species I have made use of specimens from the Huntsville beds. 

 After my plates were printed I obtained a large series of specimens from the original locality 

 in Monroe County, by which the facies and relations of the species are well established. 



While in its interbrachial structure and length of rami the species is of the 0. ramn- 

 losus type, it may be considered an intermediate form between that and the exsculptus group, 

 having some of the characters of both. This intermediate position is emphasized by the 

 variable number of primibrachs; out of 13 specimens from the type locality, and all from the 

 same colony, 5 have four IBr as in exsculptus, and 8 have three as in ramulosus. Like 

 exsculptus it has a slender, tapering form, delicate ramules in clusters with short intervals 

 between them ; whereas in the direction of ramulosus it has a decided increase in the relative 

 length of the rami, and there is a single large interbrachial plate distinctly outlined and 

 firmly anchored in the axil, without trace of any others, but with indications of a finely plated 

 perisome succeeding it. It lacks, however, the profuse development of plates in the higher 

 axils which is so prominent in ramulosus. In well-preserved specimens there is a tendency 

 to develop small nodes upon the higher axillaries. 



The species has a well-defined stratigraphic position, and leads up to O. pulaskiensis 

 of the later Kaskaskia beds, in which all trace of resemblance to exsculptus has been lost. 

 It may also be considered as the successor of the more massive 0. magnus, the type of which 

 is from the same region but in the true St. Louis beds below the Renault horizon ; that 

 species is a similarly intermediate form, but the difference from this in massiveness of the 

 rays is constant throughout a good series of specimens. 



The species described by Meek and Worthen as Forbesiocrinus semiovatus, and after- 

 ward referred by them to Taxocrinus, proves to be an Onychocrinus; it was derived from 

 somewhat lower strata in Hardin County, Illinois. The type specimen (PI. LXXIII, fig. 7) 

 is a young individual in poor condition ; it may possibly be a variety of this species, but it is 

 insufficient for specific determination. 



Types. The original is in the Illinois State Museum at Springfield ; the others figured 

 are in the author's collection. 



Horizon and locality. Lower Carboniferous, Renault formation in lower part of Kas- 

 kaskia Group ; Monroe County, Illinois, and Huntsville, Alabama. 



1 Smithsonian Misc. Coll. vol. 63, 1913, p. 15. 



