THYSANOCRINID^. 211 



Distribution. — Only known from the Niagara group of America. 



T?/pe of the genus. — Si]jJi07iocrinus armosus (McChesney). 



Remarhs. — This genus, by its asymmetrical form, is closely allied to 

 Lampterocrinus^ from which it differs as stated under that genus. 



S. A. Miller, in proposing the genus, described it as having three infra- 

 basals. We have examined more than eighty specimens, from the collections 

 of Mr. W. C. Egan of Chicago, and Mr. F. A. Greene of Milwaukee, and 

 satisfied ourselves that they all have five plates in the proximal ring. 



In studying the figures, it must not be overlooked that most of them 

 represent natural casts of the internal surface of the test, the plates them- 

 selves having been dissolved by chemical action, and that on these all eleva- 

 tions represent depressions of the inner floor. Only in the figures made 

 from casts in the natural moulds does the surface represent the true external 

 surface of the plates. 



Siphonocrinus armosus (McChesney). 

 Plate XIX. Figs. 3a, h, c. 



1859. Uucah/ptocrinus armosus — McChesney; Descr. New Palseoz. Foss., p. 95. 



1867. CO Grlyptocrinus armosus — McChesney; Trans. Acad. Sci. Chicago, p. 23, Vol. I., Plate 7, Figs. 



^a, b. 

 1881 (?) Glyjitaster armosus — W. and Sp. ; Revision Palseocr., Pari II., p. 196. 

 1882. Olyptocrinus armosus — Whitfield; Geol. Surv. Wis., Vol. IV., p. 284, Plate 16, Pig. 11. 

 1888. Siphonocrinus armosus — S. A. Miller; Amer. Geologist, Vol. L, p. 264. 



Syn. Glyptocrinus siphonatus — Hall ; Rep. of Progr., Geol. Surv. Wis., p. 22 ; and 20th Rep. 

 N. York State Cab. Nat. Hist., Plate 10, Pig. 11. 



A large species, only known from internal casts. Calyx subovoid. Dorsal 

 cup generally higher than the ventral disk, its sides decidedly convex, sub- 

 globose ; section across the arm bases slightly lobed. 



Infrabasals small, forming a shallow cup. Basals as large as the first 

 eostals, the upper angles unusually sharp, exceptionally touching the inter- 

 brachials. Eadials larger than the eostals ; the first eostals larger than the 

 second, and hexagonal. Distichals and palmars in series of two each. Arms 

 four to the ray, their structure unknown. The plates of the regular inter- 

 brachial spaces generally consist of four rows, arranged : 1, 2, 2, 3 ; the first 

 plate as large as the first costal. Anal interradius considerably wider, greatly 

 bulging from below the arm bases; the first anal higher than the radials, 

 succeeded by three plates in the second and third rows, and numerous irre- 

 gular, rather large plates at the disk. The latter plates together form 

 a trumpet- shaped inflation, apparently representing a subtegminal anal tube^ 



