70 PALAEONTOLOGY OF NEW YORK. 



Eyes small, remarkably elevated. Visual surface lunate, sub-semi-cylindrical, 

 homo-corneal. 



Thorax convex, arched upon the axis, depressed and flattened upon the pleurae. 

 Composed of nine segments. 



Axis having less than one-third the width of the body; margins sub- 

 parallel. 



Upon the pleura the elevated limb of each segment is produced into a long 

 marginal spine, sometimes equaling the width of the thorax. In front of 

 this spine, upon each segment, is a short, bifurcate marginal spine, which is 

 visible only when the posterior spine is broken away. 



Ptgidium short, lunate. Anterior margin straight. 



Axis abruptly terminating just within the posterior margin in two nodes 

 or tubercles. In addition to the strong articulating ring, it bears a single 

 annulation which extends across the pleune and is produced at the margin 

 into long spines. Anterior to this pair of spines are three pairs, of which 

 the first is shortest, and belongs to the articulating ring. There is also a 

 single pair on the terminal border, making in all ten marginal spines upon 

 the pygidium. 



Pleura flat. 



Surface ornamented with tubercles, which are crowded upon the lobes of 

 the glabella, and irregularly disposed upon the cheeks. The segments of 

 the thorax each bear a single row of strong tubercles, there being from two 

 to four longitudinal rows upon the axis, and two longitudinal rows upon the 

 pleura;. Upon the pygidium there are tubercles on the annulation, at the 

 bases of the marginal spines, and at the termination of the axis. 



Dimensions. The smallest individual observed has approximately a length 

 of ID mm., a width of 6.5 mm., and the largest a length of 21 mm. and a 

 width of 14 mm. 



There is a similarity in the more general features between 

 this species and Acidaspis tuberculatus, Conrad, from the Lower Helderberg. 



