OWEN AND SHUMARD'S FOSSIL CRINOIDEA. 59 



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 connection 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 encrinital layers of the sub-carboniferous limestone at Burlington. 

 It was also found, holding a similar geological 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. 



Platycrinus corrugatus. (New sp.) 



PL 7, fig. 2, a, b, c, d, e. 



Body cup-shaped and somewhat pentagonal in its outline. 



Basal plate pentagonal, fi.attened, concave in the centre and slightly convex towards 

 the outer margin. The point for the articulation of the superior columnar joint is 

 circular, moderately large, finely striated in radii, and exhibits a small round central 



