382 PROCEEDINGS OF THE ACADEMY OF 



as the superior edge, and excavated down nearly one-fifth the length of the 

 plate its lower edge projecting distinctly outward. Anal plate hexagonal, as 

 long as the radials, but a little narrower, about twice as high as wide. 



Each radial plate, as well as the anal piece, has an obscure rounded ridge 

 extending down the middle to its base, so as to give the body a slightly sub- 

 lifixagomtl outline when viewed from below. These prominences are also con- 

 tinued on. down upon the basal pieces, gradually becoming obsolete as they 

 converge toward the narrow lower extremity. 



. -Length of the body to the summit of the first radials, 1-31 inches; breadth 

 a.bout 1-16 inches; height of base 0-58 inch; breadth of do. above, 0-76 in. 



Locality and position. Cedar Creek, Warren County, Illinois. Burlington, 

 Limestone of subcarboniferous series. 



Dichocrinus (Pterotccrinus*) crassds. Body below the arms of medium size 

 basin-shaped, or more than twice as wide as high, widening rapidly upwards 

 from the base, and composed of thick, apparently, smooth plates, which are 

 very slightly convex, and connected by linear sutures. Base comparatively 

 large, about four times as wide as high, distinctly concave below, and widening 

 upwards from its rounded lower margin ; subcircular in outline, being scarcely 

 impressed at the sutures ; columnar facet round, coneave, and about one-fourth 

 as wide as the base. First radial plates broader than long, widening regularly 

 upwards from the base, and all concave on the upper side, which is longer than 

 either of the others ; four of them quadrangular, and one on the anterior side 

 probably subpentagonal, from the presence of a fifth obscure angle at the 

 middle of the under side. Second radial pieces very small, about twice 

 as wide as long, subtrigonal in outline, and supporting on their superior 

 sloping sides, (which are a little concave,) the two middle arms, while the two 

 lateral' arms rest partly upon their wedge-shaped lateral extremities, and in 

 part directly upon the first radials. The anal piece is rather more than half as 

 large as the first radial plates, a little longer than wide, (the lateral margins 

 being nearly parallel or slightly convex in outline,) and presenting a subquad- 

 rangular form, with a fifth obscure angle at the middle of the base. 



* This and the first of the following species, evidently belong to the same group as the 

 species upon which Mr. Lyon proposed, in the third volume of the Geological Pur- 

 vey of Kentucky, to found a new genus under the name of Asterocrinus (Pterolocnnus, 

 Lyon & Casseday). The species of which it is composed present marked differences 

 from what are regarded as typical species of Dichocrinus in the old world, and we have 

 little doubt in regard to the propriety of separating them from that genus. As there are, 

 however, some differences of opinion, amongst paleontologists, in relation to the limits 

 of the genus Dichocrinus, we have concluded to place our species in the group 1 teroto- 

 vrinus, using tne name provisionally, however, for the present, in a subgenenc sense. 



Judging from the few species of this group we have yet seen, we are led to think the 

 formula given by Messrs. Lyon and Casseday, in their paper published in the American 

 Journal of Science, (January, 1859,) should be modified somewhat. In one of our species 

 there is a minute triangular piece resting upon the middle of each first radial, and 

 partly supporting on its superior sloping sides the two pieces regarded in Messrs. Lyon 

 and Casseday's formula as the second radial. In another of our species this minute 

 piece is represemed by a larger one of the same form, which undoubtedly bears the re- 

 lations to the other parts, of a dwarfed, but true second radial; while those resting upon, 

 and partly upon it, assume the character of secondary radials and free arm pieces. 

 Hence we think this minute piece (which is sometimes wanting), however small it may 

 be, should always be regarded as a rudimentary second radial piece. If we are correct 

 in these views, the formula of this group should be stated as follows : 



Basal pieces, 2. 



Radials 1 or 2X5, the second being very small, minute, or sometimes obsolete. 



Secondary radials. 1 X 10, or wanting; the first free arm pieces sometimes resting 

 partly on the second and partly on the first radials ; and where the small second radia! 

 and the secondary radials are wantinsr, all resting directly upon the first. 



Arms, 20. 



Interradials unknown. 



Intcrbrachia] appendages 5, large, and more or less expanded. 



[Sept. 



