332 NORTH AMERICAN INDEX FOSSILS. 



5. S. capsa Clarke. Devonic. 

 Beak somewhat excentric; anterior end curved to less radius 



than posterior ; concentric striation regular. 

 Hamilton of New York, etc. 



III. Leaia Jones. 



Carapace angulated by diagonal ridges which run from near the 

 beak to the lower margin. Carbonic. 



6. L. tricarinata (Meek and Worthen). (Fig. 1650.) Carbonic. 



I 

 I 



Fig. 1650, Leaia tricarinata, X Yz- FiG. 165 1. Frotocaris niarshi, X Yt.- 



(Ind. Surv.) (After Walcott.) 



With three carina-like smooth ridges, the anterior curved, the 

 dorsal ones defining an escutcheon-like area. 

 Coal Measures of Indiana and Illinois. 



7. L. leidyi Jones. Carbonic. 

 Ridges beaded, anterior one straight; no dorsal one; concentric 



lines more distant and fewer than in L. tricarinata. 

 Pottsville of Pennsylvania. 



IV. Frotocaris Walcott. 

 Carapace univalve, with no evidence of a dorsal suture; abdomen 

 of many segments and a single pair of tail spines. Cambric. 



8. P. marshi Walcott. (Fig. 165 1.) Cambric. 

 Carapace bent downward on the sides, with 31 segments extend- 

 ing out beneath it ; surface smooth ; no evidence of eyes. 



Lower Cambric of Vermont. 



