CONTRIBUTIONS TO PALAEONTOLOGY. 135 



to the periphery, making six in all. A somewhat strongly elevated 

 aperture is situated in the depression corresponding with the 

 anal series. Basal plates small, extending but little beyond the 

 cicatrix of the column. First and second radial plates about equal 

 in size, the latter regularly hexagonal. Third radials smaller, 

 heptagonal; supporting on each upper sloping face (in the anterior 

 and postero-lateral rays) a hexagonal or heptagonal supraradial 

 plate, with brachials on each of its upper faces, giving four arm- 

 bases to each of these rays; while in the antero-lateral rays they 

 support the brachials, giving only two arms : this makes the 

 formula, 



4 



|~|- = 16 arms. 



Interradial series consisting of eight to ten medium-sized plates 

 and three to five smaller ones, which unite with those of the 

 dome : the first is largest and hexagonal, supporting two hexa- 

 gonal plates in the second range, three in the third range (one of 

 which is hexagonal and two pentagonal), two in the fourth range 

 of unsymmetrical form, and above this the smaller ones irregular- 

 ly placed. 



Anal area much larger than the interradial area, composed of twenty 

 plates (more or less), the first about equal to the first radials, and 

 supporting three in the second range; above this, irregularly ar- 

 ranged. In the four-armed rays, there are about three interbrachial 

 plates; and in the two-armed rays, one plate. Arm-bases spread- 

 ing in compressed lobes around the margin of the cup formed by 

 the interbrachial constriction of the dome. Dome composed of 

 numerous polygonal plates, which are much smaller than the 

 body-plates. 



Surface of body-plates marked by fine radiating strise. In old 

 specimens, the plates of the lower part of the calyx are thickened 

 just within the margins. 



This species differs from others in the deep constriction of the summit, 

 and in the strong spines; while its extremely depressed form is likewise 

 characteristic. 



Geological formation and locality. In the shales of the Hamilton group: 

 Western New- York. C. A. WmxE, collector. 



