400 THE CEINOIDEA CAMERATA OF NORTH AMERICA. 



lower faces convex. Second costals pentangular, in width and length resem- 

 bling the first. Distichals varyhig from two to three, owing to the number of 

 arms in the specimen ; smaller than the costals. When there are four arms 

 to each ray, which probably is the normal number, they consist of 2 X 2 

 plates, and these are followed by 2 X 2 palmars ; when, however, which is 

 quite frequently the case, the anterior ray has only two arms, it has three 

 successive distichals in both divisions. Specimens with three arms in the 

 anterior ray are comparatively rare, in this case the one division has three 

 distichals, the other two. Palmars short, in contact laterally, and rounded 

 like arm plates, leaving longitudinal depressions at their sides. Arm facets 

 large, directed outward ; the ambulacral openings elongate, almost equidis- 

 tant, the interspace between the two posterior rays being a little the widest. 

 Respiratory pores large, separated from the ambulacral passages by thin par- 

 titions, which frequently are not preserved. Arms eighteen to twenty, stout, 

 long, incurving and biserial. The lower portions of the plates are rounded, 

 and every third plate of both series is extended to form a short lateral spine; 

 the upper portions graduall}^ grow flat, and increase in width to twice that 

 at the bottom, but do not attain that sharp knife-like edge observed in some 

 species of this genus. Interbrachials from one to three; the first very large 

 rising to the top of the first distichals ; the upper plates, when present, very 

 minute. Anal piece higher than the radials ; succeeded by three large plates, 

 and these sometimes by one or two small ones. Ventral disk depressed 

 hemispherical ; the plates highly convex or nodose. Posterior oral strictly 

 central, fully twice as large as any of the other plates, and surmounted by 

 a hioh node. The four other orals and the radial dome plates are consider- 

 abl}' larger and more prominent than the interambulacral pieces, some of 

 which are quite small. Anal tube excentric and very slender ; its length not 

 Icnown. Column near the calyx composed of high joints, angular along theii? 

 edges. 



Ilorizoti and Localittj. — Lower Burlington limestone, Burlington, Iowa, 

 and Lake Valley, New Mexico. 



Tt/pes in the Museum of Comparative Zoology, and in the University 

 Museum at Ann Arbor. 



Remarks. — This species was described by Hall as having twenty arms. 

 Among twenty-one specimens in our collection, which all undoubtedly be- 

 long to it, there are ten with twenty arms ; five have nineteen, and six but 

 eighteen. The deficiency always occurs in the anterior ray. 



