FROM THE PALAEOZOIC ROCKS OF INDIANA, KENTUCKY AND OHIO. 



453 



first radials. The inferior extremity of the false basals are about as thick as the width of 

 the lowest zone forming the atlas; they arc joined by a crcnulated surface to the upper disk 

 of the column, thus completing it in a rounded hemispherical knob, composed of the two 

 sets of pieces described, the whole completely filling the lower part of the depression at the 

 base of the calyx of the animal. 



The pieces terminating the column are divided like the true basal pieces ; they form a 

 similar cup, not notched, as are the summit of the true basals, but squarely truncated and 

 crcnulated at their base, like the pieces composing the column below the body. 



The pieces rounding the top of the column are frequently found attached to the body, 

 completely filling the basalar cavity. They can generally be distinguished from the upper 

 fiat disk of the column, by the difference in the texture of the two classes of pieces, and 

 are usually seen as two rings, one within the other. 



All the deep, basal-pitted Dolatocrinus known, arc; attached to columns, the summits of 

 which are formed in the manner of those described above. No columns of the class de- 

 scribed have been found attached to any species of Hadrocrinus. The basalar pits, and the 

 rounded indentation in the true basal pieces, are formed in the same manner in Hadro- 

 crinus as those of Dolatocrinus, to which they have been found attached. 



Several species of Dolatocrinus, to which this form of the head of the column is known 

 to belong, have the column below the atlas formed of alternate broad and narrower disks. 

 The muscular attachments are not so distinctly marked upon the exterior surface of the 

 atlas, as in the large species J), glyptua (Hall's species), to which the columns particularly 

 described belongs. No opinion is offered in relation to the particular use of the head of 

 this class of columns of crinoidea. 



It is proposed to distinguish the pieces forming the exterior covering of the head of the 

 column, false basals; and the whole dome-like termination of the column, the atlas. 



Genus ACTINOCRLNUS. Miller, 1821. 



{Nat. Hist. Crinoipea.) 



ACTINOCRINUS PENTASPINUS, n. 8. 



PI. XXVII, Figs, d, d 1. 



Body, when divested of the summit covering, sub-cylindrical; slightly contracted beneath 

 the arm-bases; base squarely truncated parallel to the arm-bases; the united basal pieces 

 are pentangular; a little larger than one of the first radials; a piece of the column at- 

 tached is small and round, formed of circular pieces of variable diameter, the larger pieces 

 thinning from their junction with the smaller ones, and terminating in a sharp edge. The 



