OWEN AND SHUMARD'S FOSSIL CRINOIDEA. 61 



Column, arms, and abdominal plates undiscovered. This species is easily- 

 distinguished from P. IcBvis, 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 sub-carboniferous limestone at Burlington. 



Genus DICHOCRINUS, Munster. 



Basal plate hexagonal, composed of two pieces ; superior plates six, five of which 

 support the arms. 



This genus was established by Count Munster 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 Platijcrinus 

 elongatus, (Phillips), has in a more recent number been removed to the genus under 

 consideration, to which in the number of its superior 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 investigations. We have inspected a laro-e 

 number of specimens from the carboniferous rocks of various localities in Kentucky, 

 Indiana, Illinois, and Iowa, and have invariably found a bipartite basal plate, 

 accompanying six superior plates, and vice versa. 



Dichocrinus ovatus, (New sp.) 

 PI. 7, fig. 9, a, h. 



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 granules, 

 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 



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