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NORTH AMERICAN INDEX FOSSILS. 



Lower part of Magnesian limestone (Middle Cambric) of New 

 Jersey. 



XXIII. Agraulos Corda. 



Border of cephalon broad. Eyes small. Thoracic segments i6. 

 Pygidium with three annulations to the axis. Cambric. 

 59. A. quadrangularis (Whitfield). (Fig. 1569, ^.) Cambric. 



Neck ring with cylindrical spine ; cranidium subquadrangular. 



Middle Cambric, Braintree slates of Massachusetts. 



XXIV. Strenuella Matthew. 

 Differs from Agraulos in its elevated glabella, broad groove 

 across the shield in front of the glabella, and in the long eye lobes. 

 Neck ring with a broad, triangular, backward pointing projection, 



Fig. 1577. Solenopleura 

 acadica, restored. ( After 

 Matthew.) 



Fig. 1578. Ptychaspis miniscaensis^ cephalon 

 and thoracic segment. (After Hall.) 



Pleura strongly angular dorsoventrally (Fig. 1569, ^). Lower 



Cambric. 



60. S. strenua (Billings). (Figs. 1569,/, ^; 1576,/.) Cambric. 



Surface smooth. (Type of genus.) 



Lower Cambric (Etcheminian) of Massachusetts and New- 

 foundland. 



XXV. Ptychaspis Hall. 



Cephalon broad. Fixed cheeks wide, depressed-convex. Gla- 

 bella generally parallel-sided, convex, transversely lobed, prominent 

 in front; eyes anterior to middle. Frontal limb narrow. Free 



