750 THE CKINOIDEA CAMERATA OF NORTH AMERICA. 



spine. The sjjines, which bristle upward and outward, are acicular, very- 

 long and slender, varying in length from ten to twelve mm., somewhat 

 thicker near the proximal end, and slightly pitted at the bottom. Column 

 round and comparatively thin. 



Horizon and Locality. — Chemung group, Ithaca division ; Ithaca, N. Y. 



Types in the collection of Prof. Henry S. Williams at Cornell University. 



Remai-hs. — The description was made from natural moulds formed in 

 the rock after disintegration of the calcareous test, and from gutta percha 

 casts therefrom. 



Arthracantha punctobrachiata Williams. 

 Plate LXXVI. Figs. 2a, b. 



1883. Williams; Proceed. Amer. Pbilos. Soc. (April), pp. 83 and 86. 



1885. W. and Sp.; Kevision, Paloeocr., Part III., p. 119. 



18S7. Whiteates ; Contr. to Canad. Pateout., Vol. I., p. 93, Plate 13, Figs. 1, la. 



Sjn. Pla/j/criuus (V) puiictobrachiatus Hall (figured by Hall 1872, in Bull. 1. N. T. State Museum 

 Nat. Hist.). 



Syn. Eystricrims Carpenieri Hinde, 1885 ; Ann. and Mag. Nat. Hist, p. 163, Plate 4. 



Dorsal cnp obconical ; more rapidly spreading at the basals than at the 

 radials; the latter somewhat inflected at the upper ends. Ventral disk 

 hemispherical, flattened in the central part. Plates without ornamentation, 

 but thickly covered by minute spine-bearing tubercles, which, when well 

 preserved, resemble small cones truncated at the upper end. The spines 

 are elongate, cylindrical, and measure from fifteen to thirty-five mm. in 

 length; they have a short neck at the proximal end, and taper distallj'. 

 The distribution and arrangement of the tubercles is quite irregular; they 

 are, as a rule, more numerous upon the basals and radials, but the number 

 varies even among corresponding plates of the same individual. There are 

 but very few upon the distichals and interbrachials, but they are crowded 

 along the median portions of the disk, where, according to Hinde, they are 

 shorter and comparatively thicker than at the arms. 



Basals large, almost of uniform size; the suture lines indistinct; tlie 

 lower end truncated to the width of the column. Radials somewhat irregular 

 in form, the two facing the anal plate narrower at the lower face than at the 

 upper, and narrower throughout, the others almost rectangular, and nearly 

 as long as wide. The upper faces are straight, except the median part, 

 which is slightly excavated for the reception of the costals ; the outer faces 



