FROM THE CARBONIFEROUS LIMESTONE OF IOWA. 



589 



PLATYCRINUS CORRUGATUS. (N. S.) 

 (Tab. V. a, fig. 2, a, b,c, d, c.) 

 Body cup-sliaped and somewhat pentagonal in its outline. 



Basal plate pentagonal, flattened, 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 perforation corresponding to the columnar canal ; near the 

 centre of the plate is the commencement of three furrows, which proceed to the edges of the plate, dividing 

 it into three unequal parts. The surface is finely corrugated ; in some specimens gathered into imbrica- 

 ting folds arranged in concentric lines, conformable to the shape of the plate. 



The superior plates are five; broad, subhexagonal, and widen slightly from below upwards. The exca- 

 vations for the attachment of the arms are harp-shaped, large, and striated around the margin, with a 

 deep angular notch at the summit. The vertical diameter of the excavations is equal to about one-half 

 of the length of the plate. The surfaces of the plate arc finely corrugated, sometimes gathered in imbri- 

 cating folds, finely striated, and disposed in lines parallel to the lateral and inferior margins. The articular 

 edges are striated, as are those of the basal plate ; and when connected together a deep angular gutter is 

 formed at each of the sutures. 



Arms. — We are only acquainted with the first joint of the arms of this species, which remains attached 

 to a single specimen. It is cuneiform, the upper edges being bevelled so as to form two facets finely 

 striated on the margin for the articulation of two series of arm-plates'; the external surface is corrugated. 



Column. — We have recognised merely a small fragment, consisting of the three superior plates of this 

 portion of the animal, which is preserved in the beautiful specimen represented in fig. 2, a. They are 

 circular at the basal plate, but become slightly elliptical as they recede from it ; thin and of unequal 

 diameter; each joint is sharply carinated on the side and presents a serrated appearance, from the elevated 

 strife on the articular surface of one plate fitting into the intervals between the strife on the other. 



Formation and localities. — This exceedingly neat and elegant species is rather rare, and occurs with 

 the preceding in the subcarboniferous strata at Burlington, Iowa. We have not met with it at any other 

 locality. 



PLATYCRINUS BURLINGTONENSIS. (N. S.) 

 (Tab. V. a, fig. 5.) 

 Body cup-shaped, with a slightly pentagonal contour. 



Basal plate pentagonal, flattened, smooth, moderately thick, convex towards the margins, and depressed 

 in the centre, with a circular facet for the articulation of the supra-columnar joint. 



Superior plates five; subhexagonal, smooth, slightly convex, and a little wider at their superior than 

 at their inferior margins. The breadth of the plates is about one-fourth greater than the height. The 

 articulating surface for the attachment of the arm is, in part, on the superior edge of the plate, and is 

 somewhat rcniform and shallow; in its longest diameter equal to about one-third the breadth of the plate. 



Column, arms, and abdominal plates undiscovered. This species is easily distinguished from P. Isevis, 

 to which it is closely allied, by its more depressed form and flattened basal plate. 



Formation and locality. — It is rare, a single specimen only having been obtained from the quarries in 

 the subcarboniferous limestone at Burlington. 



DICHOCHRINUS. MUnster. 



Basal plate hexagonal, composed of two pieces; superior plates six, five of which support the arms. 



This genus was established by Count Miiuster to receive a crinoid possessing the above characters, viz., 

 Dichocrinus radiatus, from the carboniferous limestone of Tournay, Belgium. Since then, other species 

 have been discovered and published by authors. A species from the mountain limestone of Yorkshire is 

 described by Messrs. Austen, under the name of D. fusiformis (Monog. Crinoid., p. 47, pi. 5, fig.6, a-d) } 

 and another figured in an early number of the same work as Platycrinus elongatus (Phillips), has in a 



