404 NORTH AMERICAN INDEX FOSSILS. 



furnished dorsally with two large, compound (facetted) lateral 

 eyes and a pair of median ocelli, and ventrally with six pairs of 

 legs, the first preoral and chelate, the others non-chelate. Ab- 

 domen of thirteen joints, the six anterior segments bearing on 

 their under side, five pairs of broad, leaf-like appendages prob- 

 ably comparable to the " gill-books " and operculum of the Xipho- 

 sura. The posterior seven segments, including the telson, are 

 without appendages. The inner margins of the legs are fur- 

 nished with stout spines which serve as teeth. The last pair of legs 

 is usually large and somewhat flattened, and ends in an oval plate. 

 This ''paddle'' may have been used for swimming or for burying 

 in the mud or for purpose of anchoring. On the under or ventral 

 surface of the first two abdominal segments, is the genital oper- 

 culum, a pair of plates meeting medially, with a median lobe 

 attached which differs in the two sexes. Eurypterids were prob- 

 ably mostly fresh or brackish water animals, their remains being 

 relatively rare in typical marine* strata. 



Literature. 

 1859-1888. Hall, J., and Hall, J., & Clarke, J. M. Pal. N. Y. , vols. 

 3 and 7. 



1900. Beecher, C. E. Kestorsition of S/y/onurus /aam/n^s, etc., Am. 

 Journ. Sci., Vol. X., pp. 145-150, pi. i. 



1901. Beecher, C. E. Discovery of Eurypterid Remains in the Cam- 

 brian of Mo., A. J. S., 4th Ser. vol. 12, pp. 364-66, pi. VII. 



1903. Sarle, C. J. A New Eurypterid Fauna from the Base of the 



Salina of Western N. Y., N. Y. State Mus. Bull., 69, pp. 1 080-1 108. 

 1907. Clarke, J. M. The Eurypterus Shales of the Shawangunk 



Mountains in Eastern N. Y., N. Y. State Mus. Bull., 107, pp. 295- 



310, pis. 1-8. 

 1909. Clarke, J. M., and Ruedemann, R. Monograph of the 



Eurypterida Memoir, N. Y. State Museum (in preparation) abstract 



N. Y. State Mus. Bull., 133, p. 35. 



VII. Strabops Beecher. 

 Cephalothorax comparatively larger and wider than in Euryp- 

 terus, the eyes further forward, nearer together and more oblique ; 

 twelve abdominal somites besides the telson as in Eurypterus. 

 (Clarke and Ruedemann, only 11 shown in figure.) Cambric. 

 II. S. thatcheri Beecher. (Fig. 1702.) Cambric, 



