524 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



the three plates above succeeded by three plates, and numerous other plates 

 irregularly arranged in rows of from five to six pieces each, which grow 

 smaller as they approach the tegmen. Upon the ventral disk there is a well 

 defined anal ridge, which ends in a subcentral proboscidiform protuberance 

 with the anus at the upper end. Interdistichals consisting of a rather large 

 plate resting between the sloping upper faces of the first distichals, and six 

 or more rows of two plates each, the upper ones resembling the plates of the 

 tegmen. Tegmen almost flat ; composed exclusively of small polj'gonal 

 plates. In the casts there appear upon the surface ten well marked ridges, 

 which represent grooves in the test for the reception of the ambulacra. 



Horizon and Lomliti/. — Upper part of Niagara group ; near Chicago, Ills. 



Remarhs. — It probably occurs also at Waldron. Among the specimens 

 which we refer to it there is one in which large portions of the arms are pre- 

 served. They branch at about 18 mm. above the calyx, and one of the 

 branches again at about 25 mm. above the first bifurcation. Whether 

 any additional bifurcation takes place in this species, cannot be ascertained, 

 as the arms are broken at 65 mm. from the calyx. The arms are composed 

 of two rows of short, transverse pieces, very regularly arranged. 



Periechocrinus necis (TVistch. and Marcy). 

 Plate L. Figs, la, h. 



1865 Megistociiims necis — Wincueil and Makct ; Mem. Bost. Soc. Nat. Hist., p. 110, Plate 2, Fig. 6. 

 18S1. Saccocrimis necis — S. A. Milleii; Jouru. Ciuciu. Soc. Nat. Hist., Vol. IV., p. 172, Plate 4, Figs. 



3, 3ff. 

 1885. Periechocrinus necis — "W. and Sp. ; Revision Palseocr., Part III., p. 106. 



A shorter species than the preceding. Cnlyx pjn-iform, not much higher 

 than wide ; obconical to the top of the second radials ; greatest width a little 

 above the second costals, whence it contracts rapidly to the arm bases, at 

 which the calj'x is reduced to two thirds its former width. Surface of plates 

 smooth, with slight angularities following the radials, giving to the cross- 

 section a somewhat pentangular outline. 



Basals of medium size, move rapidly spreading than the parts above. 

 Radials and costals occupying two thirds the height of the calyx, nearly of 

 equal size, longer than wide ; their upper and lower faces wider than usual 

 in this genus. Distichals two ; the first plate one third smaller than the 

 costals ; the second as much as one half. Palmars two or more, small, and 

 between them are interposed 1, 2, 2, small interdistichals. Interbrachials 



