590 



NEW SPECIES OF CRINOIDEA, 



more recent number been removed to the genus under consideration, to which in the number of its supe- 

 rior plates, and the form of the basal plate, it more properly belongs. 



In the United States, we are acquainted with five species appertaining to the genus Dichocrinus, all of 

 which are from the carboniferous strata of the Mississippi Valley, and differ from any known European 

 forms. 



The opinion advanced by Messrs. Austen, that all the Crinoids occurring in the carboniferous limestone, 

 with six superior plates, will be found to have a bi-partite basal plate, is fully borne out by our investiga- 

 tions. We have inspected a large number of specimens from the carboniferous rocks of various localities 

 in Kentucky, Indiana, Illinois, and Iowa, and have invariably found this association to prevail where the 

 superior plates rest immediately on the basal plate. 



DICHOCRINUS OVATUS. (N. 8.) 

 (Tab. V. a, fig. 0, a, b.) 



The body of this beautiful species, when deprived of its arms, is of an ovoidal form. 



Basal plate bi-partite, cup-shaped, hexagonal, thin, rounded ; upper margin undulated to receive the 

 orbicular edges of the superior plates ; circle of attachment for the column slightly excavated, small, 

 circular, and surrounded by a row of small and short spines. The surface of the plate is ornamented by 

 depressed granulfe, arranged in rows, some of which commence at the margin of the excavation for the 

 supra-columnar joint, and proceed to each angle of the plate ; other lines run parallel with the undulating- 

 edges of the plate, forming a series of hexagons, one within the other, giving to the whole surface an 

 exceedingly neat and elegant appearance. 



Superior plates six, of which five are elongated, quadrilateral, rounded at the inferior edges, and sup- 

 port the arms ; one is trapezoidal, wide, slightly angulated below and narrow above. The articular sur- 

 faces for the arms, situated on the superior edges of the five arm-bearing plates, are moderately large, 

 very slightly excavated, striated on the margin, with a transverse ridge near the centre. All the supe- 

 rior plates are finely corrugated on the external surface. 



Arms. — Several joints of the arms remain attached to one of the superior plates, in the only specimen 

 we have been able to procure of this species. The first joint is of a rectangular form, and supports a 

 cuneiform joint, on the bevelled edges of which is the commencement of two series of smaller plates. 

 Each of these joints is distinctly corrugated on the dorsal surface. 



Dimensions. — Height, nine lines; greatest width, 8 lines; diameter at summit, six lines. Height of 

 pelvis, three lines ; height of superior plates, five lines. 



The column is undiscovered. 



This is one of the most beautiful species with which we are acquainted. It was obtained from the 

 quarries in the carboniferous limestone at Burlington, where it is exceedingly rare. 



DICHOCRINUS STRIATUS. (N. S.) 

 (Tab. V. a, fig. 10, a, b.) 

 Body globose, and composed of thin plates. 



Basal plate hexagonal, bi-partite, hemispherical, superior margin undulating; circle of attachment for 

 the column small, slightly excavated, finely striated in radii, with a minute round perforation in the 

 centre. The surface is marked with permanent thread-like strife, some of which begin at the excavation, 

 others originate near the middle of the plate, and radiate to the superior margin. 



Superior plates six, five of which bear the arms; these are quadrilateral, somewhat elongated, wider 

 inferiorly than at the summit, with the articulating surface for the arms, as in the preceding species, 

 situated on their superior edges. The non-ray-bearing plate is trapezoidal, wide, slightly angular at the 

 base and narrow above. The striae on this series of plates range in lines parallel to the lateral borders, 

 and join those in the basal plate. Some of the striae are bifurcated near the upper margin, and when 

 examined with a lens, have an undulated appearance. The furrows between the strife are crossed at 

 irregular intervals by transverse threads. 



Formation anal localities. — Occurs at Burlington in the encrinital layers of the carboniferous limestone, 

 immediately above the oolitic members. It is not an abundant species. 



