402 NORTH AMERICAN INDEX FOSSILS. 



Packard) segments besides a large, thick tail spine or telson. 

 Devonic. 



8. P. eriensis (Williams). (Fig. 1701.) Devonic. 



Axial length 100 mm., width of axis five to eight mm., length of 

 telson 32 mm. ; genal spines scarcely protruding. (Type of genus.) 



Chemung of Pennsylvania. 



Fig. 1 701. Protolimulus eriensis. Diagrams of lower and upper (theoretical) 

 sides : A, cephalic shield ; ^, ? hypostoma ; C, genal spine ; D, thoracico-abdominal 

 buckler; E, telson ; g, marginal abdominal spines ; ii, longitudinal ridges of buckler ; 

 KK, portions of the gnathopodes ; Z, ? foliaceous terminations of the last gnathopodes ; 

 M, position of the mouth, X Yz- (After Simpson. ) 



Order SYNXIPHOSURA Packard. 



Cephalothorax semicircular, median axis more or less well- 

 defined; compound eyes generally present; abdomen trilobed, its 

 segments free, resembHng the thorax of a trilobite ; pleura flat and 

 extended, and generally terminating in lateral spines. 



V. Aglaspis Hall. 



Cephalothorax large, trilobed, central portion short, conical, 

 with two compressed eyes in front of it; no facial suture; pleura 

 not grooved. Cambric. 



9. A. eatoni Whitfield. (Fig. 1702.) Cambric. 



Head-shield lobed, subsemicircular, conate central portion about 

 half the length of cephalothorax; telson long and slender. 



St. Croix (Upper Cambric) of Wisconsin. 



