588 



NEW SPECIES OF CRINOIDEA, 



in the column. The surface is embellished with very prominent tubercles, usually arranged in a single 

 series, just within and parallel to the outer borders of the plate. 



The superior plates are five; broad, subhexagonal, and spreading towards the superior edges. The 

 surface of each plate, like that of the basal plate, is studded with highly-raised tubercles ; in some speci- 

 mens these are disposed with great regularity in ranges parallel to the lateral and inferior edges ; in 

 others they are scattered irregularly over the plate. The excavations for the attachment of the arms are 

 wide, shallow, and somewhat rcniform. Each excavation is striated on the margin, and crossed near the 

 middle by a transverse ridge, serving to prevent displacement of the arms. 



The column and arms have not been discovered. 



Dimensions. — Height of cup, four lines ; breadth, seven and a half lines ; height of superior plates, four 

 lines ; breadth of superior plates, five lines. This encrinitc is nearly related to Platycrinus rugosus and 

 P. tuberculatus of Miller, but is separated from those species by the more depressed form of the cup, the 

 greater proportionate breadth of the superior plates, and the larger articulating surface for the arms. 



Formation and locality. — It occurs with the preceding species in the subcarboniferous limestone at 

 Burlington, Iowa. We have likewise discovered it near Salem, Indiana, holding a similar geological 

 position. It affords us much pleasure, as a slight tribute of respect and esteem, to dedicate this beautiful 

 species to our friend Professor L. P. Yandell, of Louisville, Kentucky. 



PLATYCRINUS DISCOIDEUS. (N. S.) 

 (Tab. V. a, fig. 1, a, h.) 



The general outline of the cup of this species is that of a decagon ; and the visceral cavity is propor- 

 tionally more shallow than in any of the hitherto described species of this genus, the diameter being 

 about one inch, while the height is not quite three lines. 



Basal plate pentagonal, massive, concave towards the centre, and bordered near the margin by an 

 uneven ridge. The circle of attachment for the column is moderately large and finely striated. The 

 margins of the plate are bevelled. 



The superior plates are five; somewhat massive, subhexagonal, and increase in breadth towards the 

 superior edges. The lateral and inferior margins of each plate are bevelled, so that when articulated to 

 the basal plate and its fellows, a well-defined channel is formed at the sutures. A large portion of these 

 plates goes to form the base of the cup, and occupies the same horizontal plane with the basal plate ; being- 

 bent upwards only a short distance from the superior margin to form the sides of the visceral cavity. 

 The articulating surface, for the attachment of the arms, is harp-shaped, slightly excavated, rather large, 

 and finely striated on the margin. These surfaces are nearly perpendicular to the base, and their length 

 is about equal to the entire height of the cup. A small groove, bounded by an uneven ridge, surrounds 

 the inferior and lateral borders of each excavation. The surfaces of all the superior plates are ornamented 

 with uneven ridges, two and sometimes three of which run parallel with the lateral and inferior margins ; 

 these are crossed, at their angles of junction, by two oblique diverging ridges, which commence at the 

 lower border of the articulating surfaces for the arms; and proceed to the inferior angles of the plate. In 

 some specimens the furrows between the parallel ridges are crossed by small connecting ridges. 



The plates covering the abdominal cavity in their connexion are slightly arched, and nearly all of them 

 have a mammillary projection rising from their centres. In the specimen which we have found with this 

 portion of the animal preserved, the forms of only two of them can be made out; these are hexagonal, 

 the others are so badly preserved that their shape cannot be satisfactorily ascertained. 



The oral aperture, situated a short distance within the outer margin of this plated integument, is rather 

 large, circular, and surrounded by a slightly elevated rim, composed of small plates. 



Column and arms unknown. 



Formation and localities. — A few specimens only of this elegant crinoid were obtained from the encri- 

 nital layers of the subcarboniferous limestone at Burlington. It was also found, holding a similar geolo- 

 gical position, at Augusta, Iowa. At the latter locality we procured some detached superior plates, clearly 

 appertaining to this species, measuring nearly an inch in the long diameter. 



