size. It is followed by three larm- and elongated plates in the second 

 range that curve up out of the calicular cavity and over tlie margin 

 of the calyx and unite with the plates of the vault. 



The vault is highly convex and more or less concave in the inter- 

 radial regions. There is a large tumid plate at the apex of the vault 

 which is surrounded by eight plates, the two smaller ones of which 

 unite with the minute plates that surround the azygous orifice. There 

 is a large tumid plate over the junction of the ambulacral canals; the 

 other plates of the vault are much smaller, but they are very irregu- 

 lar in size; the smallest ones are in the regular interradial and azygous 

 areas. The azygous area is wide and covered by numerous plates. 

 The orifice is at the superior part of a bulbous swelling and sur- 

 rounded by small plates. 



This is the first fifteen-armed species ever deserilieii and may be 

 distinguished by that character alone. It is, however, to be distin- 

 guished by its general form, peculiar arrangement of the secondary 

 radials. and by the regular interradial and azygous plates. 



Found in the Keokuk group, at Keokuk. Iowa, and now in the col- 

 lection of Wm. F. E. Gurley. 



ACAKICOtRINrS KEUKUKENSIS. n. sp. 



Phde 1, Fill. 4. Ii(i>i(il rieir : Fii/. .'i, siniiniil (■/i-ir: Fn/. <i, (izijuons 

 vii'li' of siiiiir ^licclllicii. 



Species large, subpentagonal, in outline, by reason of the exten- 

 sion of the radial areas, calyx very deeply concave and vault very 

 highly convex leaving a thin visceral cavity, and. in these respects, 

 very much resembling A. /(iirciisis aliove described. Plates thick, 

 convex, and giving to the whole form a rough aspect. 



Basals form a slightly concave hexagonal disc that is substantially 

 covered with the end of the cdiuiiin. First primary radials about as 

 long as wide, and gently curve from the basal disc so as to form a 

 concave bottom for the cavity of the calyx, three hexagonal, two hep- 

 tagonal. Second primarj' radials nearly as long as wide, quadrangu- 

 lar. Third primary radials somewhat wider but not any larger than 

 the second and wholly within the concavity of the calyx, pentagonal, 

 axillary, and support on the upper sloping sides the secondary radials. 

 First secondary radials large, tumid and having the inferior end 

 within the cavity of the calyx, and the superior part curving over, 

 horizontally, in the direction of the arms. In the ray on the left of 



