Remarks on the Trenton Limestone of Kentucky. 159 



resting upon one of the sloping faces of the contiguous basals, and the 

 inner upper angle truncated, where it meets the suture, uniting the 

 large basal with the two smaller az^^gous plates, supported by its 

 upper surface. Though pentagonal, the}^ are ver}^ different in shape 

 from the other three. All these plates are very much thickened, so 

 that the central portions are much elevated above the sutures, which 

 gives the basal view a pentalobate appearance. 



Brachials — Ten, first series five, quadrangular, equal in shape and 

 size, wider than high, deeply constricted centrally, squarely truncated 

 above and below, rapidly tapering upward, so that while the lower ar- 

 ticulating face is equal to that of the radial, the upper is much shorter. 



These plates are much thickened and tumid below. The suture 

 uniting them with the radials gapes widely, leaving an exposed por- 

 tion of the upper surface of the radial as in Cromyocrinus and Eypa- 

 chycrinus. Second series, four pentagonal, higher than wide, centrally 

 constricted, rapidl}^ tapering upward, and giving rise to two arms upon 

 the upper, sloping, outer side. These plates are likewise much thick- 

 ened at the articulating extremities, and resemble, in that respect, and 

 the central constriction, the brachials of the first series. The fifth 

 plate of this series, or that opposite the az3'gous side, is quadrangular, 

 being rendered so by the single articulating surface above, giving rise 

 to a single arm instead of two. Its form is otherwise like that of the 

 others. 



Azygous Plates — Of these but three are well shown, the two lower 

 being irregularlj^ pentagonal, and resting upon the upper face of the 

 large basal as before mentioned. Above these are two or three others, 

 of which only the lower one is exhibited with any degree of distinctness 



Arms — Nine, composed of plates shaped much like the upper 

 brachials, longer than wide, quadrangular in shape, laterally con- 

 stricted, with straight sutures. As the plates are long, and the arms, 

 comparatively short, the number of arm plates is unusually small. 

 So far as can be determined the arms are without divisions. 



PinnulcE — These were few, but composed of stout and long pieces 

 like those of the arms. 



Ventral Sac — This was comparatively large, composed of heavy 

 hexagonal plates, which are thickened and raised into more or less 

 angular ridges at the sutures. This organ extended nearly to the 

 extremity of the arms. 



Column — Pentagonal, very small, the plates being alternately thicker 

 and thinner, and ornamented at the angles with small tubercles or 

 prominences. Canal, round and very small. 



