5 so NORTH AMERICAN INDEX FOSSILS. 



159. R. whitei Hall. Mississippic. 

 The largest known American species (three fourths of an inch 



broad). Calyx wider than high, deeply concave below. Plates of 

 dorsal cup strongly convex, smooth. Anus subcentral, at the end 

 of a short tube. 



Lower Burlington of Missouri and Iowa. 



LVIIL Arch^ocrinus Wachsmuth and Springer. 

 Closely similar to Rhodocrinus, but with relatively larger calyx, 

 shorter arms, and but two interbrachials in the second row while 

 Rhodocrinus has three. Ordovicic. 



160. A. pyrif ormis (Billings). Ordovicic 

 Large (about one and one half inches wide), obconical, con- 

 tracted above. Surface smooth or finely granulose. Distichals to 

 height of the sixth plate incorporated in the calyx. 



Trenton of Montreal. 



LIX. Lyriocrinus Hall. 

 Calyx depressed-globose, flattened to the middle of the R. IB 

 5, very small, concealed by the column. B 5 ; R separated all 

 around by the large IR. Costals two, 

 large. Two of the distichals enclosed 

 in the calyx. Tegmen flat, not rising 

 above the dorsal cup and arm openings 

 directed upward. Arms two to the 

 ray, rising in a straight line with the 

 sides of the calyx, simple, biserial. 

 Anus subcentral. Column round (Fig. 

 1879). Siluric. 

 Fig. 1879. ff--;7^' ^"-ly- 161. L. dactylus Hall. (Fig. 1880.) 



SIS of calyx. (After Weller.) -^ . 



Siluric. 



Center of base abruptly depressed for the reception of the 

 column. Plates finely ornamented with granules which become 

 elongate near the margins of the plates. (Type of genus.) 



Niagaran of New York. 

 162. L. melissa (Hall). Siluric. 



Usually larger than L. dactylus, with heavier arms. Basal pit 

 surrounded by a 5-angled rim. Anus subcentral, very wide. 



Niagaran of Indiana. 



