696 THE CRINOIDEA CASiERATA OF NORTH AMERICA. 



scarcely one third the width of the phates, and extending but little inward. 

 Costals subtriangular, the sloping upper faces concave, the angle sharp. 

 Distichals nearly as long as wide, not in contact laterally, each one giving 

 off two arras. Arms four to the ray, rather delicate from their origin, com- 

 posed of cuneate pieces, which interlock from the fourth or fifth piece and 

 become biserial ; their joints are long, and all more or less constricted across 

 the middle. Ventral disk moderately high, hemispheric ; its plates numer- 

 ous. The interambulacral pieces of the regular sides consist of three and 

 two plates, so far as observed, of which the middle one of the first row is 

 large ; the anal side has five in the first range, followed by numerous very 

 small, convex pieces, forming a large, well defined protuberance, which 

 encloses the anus. Orals and ambulacral plates not visible in the specimens. 



Horizon and Locality. — Lower Burlington limestone, Burlington, Iowa. 



Type in the Museum of Comparative Zoology. 



Bemar/cs. — Hall's P. cli/tis is a young specimen of P. scobina M. and W., 

 and the former name should have priority if the form had been satisfactority 

 described ; but unfortunately the short preliminary notice of it which Hall 

 gave is insufiicient for specific identification, and we must in justice to Meek 

 and Worthen accept their name. Hall's more elaborate description, which 

 appeared in 1862, is misleading, for he describes the species as having only 

 three arms to the ray, in which it would be unique among all Platycrinidte. 

 Hall's type was imperfect, and the arms, according to the figure, are uni- 

 serial throughout, as in all young specimens of Platycrinus. 



Platycrinus parvinodus Hall. 

 Plate LXVIII. Figs. 6a, h. 



1861. Hall; Descr. New Spec, of Crinoids, p. 17. 



ISSl. "W. and Sp.; ReTision, Part II., p. 73 (Proceed. Acad. Nat. Sei. Pliila., p. 247). 



A small species. Dorsal cup wider than high, broadly caliculate, slightly 

 obconical at the lower end, the sides evenly convex. Plates extremely thin, 

 marked by irregular lines of sharp, very small nodes, passing from the radial 

 facets to the lower angles of the plates, and from the columnar facet to the 

 upper angles of the basal cup, with a few similar nodes irregularly scattered 

 upon the surface. The nodes in some specimens are only visible with 

 a magnifier. Basi-radial and interradial sutures on a level with the general 

 surface of the plates. 



