m. CONTRIBUTION TO 



We doubt if A. extensus is specifically distinct from A. hulbosus, but as 

 our specimen is, doubtless, the form described by W. & Sp., we refer it to their 

 species. 



The arms are massive and composed of moderately thick joints, the rays 

 tapering but little to their extremeties. 



The left posterior ray is preserved to the fifth brachial which is not a bifur- 

 cating plate. The right posterior ray divides on the third brachial, so also the 

 right anterior, while the left anterior bifurcates on the second brachial, the an- 

 terior itself dividing on the thirteenth. Wachsmuth & Springer in their diag- 

 nosis of this species assert that all the arms bifurcate on the second brachial, 

 except the anterior ray which divides on the fourteenth or fifteenth plate. As 

 to the number of times the rays branch, there seems to be nothing fixed. It ap- 

 pears to be from five to six. The diiferences we have pointed out are of little 

 consequence. 



The arm groove is scarcely half the width of the ray. 



The calyx plates are but moderately thick, the articular facet occupying 

 the greater part of the upper edge of the radial. 



The fossil figured came from the Upper Helderberg group, at the Falls of 

 the Ohio, the horizon, doubtless, of W. & Sp.'s type, though they give "Hamil- 

 ton group below Hydraulic beds" (Encrinal Limestone, Hall). 



Collection of G. K. Greene. 



MEGISTOCRINUS CIRCULUS?, Rowley. 



Plate 54. Figs. 2, 3, 4.  



This large crinoid agrees well with the above species in some respects, but 

 differs quite as much in others. 



It is a very depressed form but with a shallow concavity extending scarcely 

 beyond the first radials. 



The ventral disk is but little convex, the calyx contracted below the arm 

 bases and all the calyx plates distinctly concave, the plate edges being ridges. 

 The anterior and two posterior rays have four arm bases each while the two lat- 

 eral have but two (each). There are ten spines on the vault, one located cen- 

 trally and one above each ray while a smaller one is at the junction of two am- 

 bulacral ridges. This would give twelve but there is one each wanting on the 

 anterior and left posterior rays. The surface of the ventral -plates and, doubt- 

 less, so of the calyx plates, is granulose. 



The base of the ventral tube is of moderate size. The periphery has a 

 pinched appearance between the arm bases. 



The specimen comes from the Hamilton group at Isaac Perry's farm, near 

 Slate Cut, Clark county, Indiana. Collection of G. K. Greene. 



