EHODOCRINID^. 259 



Allied to ArchceocrinuSy but arms uniserial and not bifurcating. Radial 

 ridges generally well developed. Infrabasals small, not visible from a side 

 view, and generally completely hidden by the column. Basals large, hexa- 

 gonal, the upper face truncated for the reception of the first interradials. 

 Eadials and second costals of similar form, but the former angular below, 

 the latter angular above. Arms ten to twenty, free from the distichals or 

 palmars according to species; they are stout, long, simple, and composed 

 of quadrangular pieces with strong pinnules. Interbrachials and interdis- 

 tichals numerous. Structure of ventral disk, and form of the anus unknown. 

 Column large and round. 



Distribution. — Both American species are from the Lower Silurian. Oeh- 

 lert described a species from France under the name Rhaphanocrinus Wachs- 

 ?mtthi, which came from the " Devonien inferieur/' but this is apparently 

 not a typical form. 



T^j)e of the genus : Bhaplianoermus subnodosus, 



Rhaphanocrinus subnodosus (Walcott). 

 Plate XI. Fig. 2. 



1883. Glyptocrinus (?) subnodosus —Yl KLf^Q>ri ; 35th Rep. N. York State Mus, Nat. Hist, p. 208, Plate 



17, Kg. 3. 

 1885. Rhaphanocrinus subnodosus — W. and Sp. ; Revision Palseocr., Part III., p. 99. 



Calyx of medium size, obconical ; interradial areas depressed. Eadial 

 ridges well defined and marked by a small node in the centre of each plate ; 

 they pass down to the middle of the radials, where they meet with similar 

 ridges from the two adjoining basals. The interradials have also a central 

 node, from which obscure ridges pass out to the margins of the plates, where 

 they meet with ridges from adjacent plates. 



Infrabasals completely covered by the column. Basals very large, fully 

 as high as wide, slightly truncate above. Eadials heptagonal, larger than 

 either one of the costals, decidedly angular below. Arms ten, rather stout, 

 simple, the lower plates incorporated into the calyx, the second and all suc- 

 ceeding ones pinnule-bearing ; they are composed throughout of compara- 

 tively long, quadrangular joints, with stout pinnules, placed wide apart. 

 Interradials: 1, 2, 3, 3, and two or more rows above. Interdistichals : 1, 2. 

 Nothing is known of the ventral disk or of the anal opening. Column 

 round, very large. 



