ACERATA—MEROSTOMATA. 403 



VL PsEUDONiscus Nieszk. 

 Oval Crustacea with relatively short cephalothorax, character- 

 ized by a broad central and more or less ill-defined lateral portion ; 

 eyes apparently absent; abdominal segments ten, the last often 

 in form of short spine ; pleura flat ; telson strong. Siluric. 



10. P. roosevelti Clarke. (Fig. 1703.) Siluric. 



Subovate, broadest in front; head shield evenly convex, about 



as long as the abdomen, apparently undivided, and without eyes; 



Fig. 1702, Aglaspis eatoni. (After Fig. 1703. Pseudoniscus roosevelti, X Z- 

 Whitfield.) (After Clarke.) 



margin narrow, flat, anteriorly produced in blunt point, posteriorly 

 crenulated on either side of axis, and produced into short, genal 

 spines; telson with strong median angulation. 



Pittsford shales (basal Salina) and Bertie waterlime of New 

 York. 



Order EURYPTERIDA Burmeister. 



Large, Crustacea-like forms, with an elongate body composed 

 of cephalothorax , a ringed abdomen, and a tail-piece (telson) ; 

 the body is covered by a chitinous skeleton which could be shed 

 as in the modern horseshoe crab. The cephalothorax is usually 



