164 I'AL.KONTOLOGY OP ILLINOIS. 



Primary radials decreasing in size in ascending order, all 

 three wider than high. First radials hexagonal and heptagonal; 

 the second quadrangular with convex sides; the third axillary. 

 Secondary radials 1x10; axillary; supporting the two main 

 divisions of the ray. Each of these plates is succeeded by two 

 pieces, of which only the inner one of each ray bifurcates again, 

 giving off two simple arms; while the outer ones, which are 

 truncate above, support a row of fixed cuneiform brachials. 

 The latter are alternately arranged, and interlock on becoming 

 free. There are six arms to the ray, or thirty in all. Excep- 

 tions to this rule are not unfrequent, some of the rays having 

 occasionally five, others seven arms. 



Arms long; moderately stout, rounded along the back; their 

 tips curved and folded inwards. They are composed of two 

 series of joints, alternately arranged. About every second to 

 fourth joint of each series is longer and bears a conspicuous 

 tooth-like node, giving two alternating rows of nodes on the 

 back of each arm. The intervening joints are very short and 

 connected vertically by waving sutures. Pinnules closely packed 

 together, contiguous. They are composed of short joints, each 

 one provided laterally with a short, tooth-like projection. 



Interradials variable in size; the first comparatively large; 

 occupying fully one-half or more of the interradial space in the 

 dorsal cup, and extending up to the middle of the third radials. 

 The first supports two very small pieces in the second range, 

 .succeeded by two still smaller ones, and others which constitute 

 a part of the ventral covering. The ventral side consists of 

 small, irregular pieces, interspersed with larger ones which are 

 tuberculous. The first anal plate is as large as the first radials 

 or larger, and generally higher. It supports two plates which 

 are smaller than the first interradial of the four regular sides. 

 The next row consists of three much smaller pieces, two of 

 them interradials, which enclose a second anal plate, with two 

 small plates above. All succeeding pieces are ventral in position 

 and form part of the anal tube. The regions above the arm- 

 has* 's arc hemispherical, not turbinate as in the allied Burling- 

 ton species, and the anal tube, at its base, is comparatively 



