44 



The vaiilt is highly convex in the central part and over the 

 ainbulacral channels and depressed toward the margin in the 

 interambulacral areas. There is a large spine bearing plate 

 in the center of the vault and one over the junction of the 

 ainbulacral canals in each of the five radial series. The other 

 plates of the vault are jiolygonal, not tumid or spinous, but 

 radiately sculptured after the manner of those forming the 

 calyx. The plates are generally small and the sculpturing is 

 so delicate and the sutures so indistinct that the artist found 

 it imiiracticable to represent these characters in the illustra- 

 tions. The azygous opening is large, subcentral, elevated and 

 surrounded, at the base, with numerous small plates. 



The general form and number of plates in the secondary and 

 tertiary series will at once distinguish this species from M. 

 expansus, and it is so diiferent from all other sixteen armed 

 spf^cics, that no comparison with any of them is necessary. 

 It is a marked and beautiful spcnnes. 



Pound in the Hamilton Group, in Clarke county, Indiana, 

 and the one figured is now in the collection of S. A. Miller, 

 while there are four specimens in the collection of J. F. Ham- 

 mell, of Madison, Indiana, and many more than that in the 

 collection of Win. P. E. Gurley, varying from one-fourth the 

 size of the s])('cinicn illusti'ated, to morti than twic(^ its dimen- 

 sions. 



MEGISTOCRINUS HEMISPHERICUS, n. Sp. 



Plate II, Fi(j. IS, basal view azygous side up; Fi(j. 19. azygous 

 side view; Fig. 20, summit view. 



Species medium size. Calyx suljhomispherical, broadly flat- 

 tened below, a little more tlian one-halt wider tlian high, not 

 constricted below the arms. Arm openings directed upward at 

 an angle of forty-five degrees. Surface in our specimens ap- 

 parently smooth, without any tumid jdates, though as our 

 specimtnis are siliciticd. tlie grauul(>s, if any, are destroyed. 

 Column large and having a large canal. The sutures are nearly 

 all obliterated, in our .specimens, above the third lu'imary ra- 

 dials rendering a technical description of the jDlates above the 

 third primary radials impracticable. 



