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NEW YORK STATE MUSEUM 



the operculum and partly by the second sternite. The part car- 

 ried by the operculum follows two subtriangular areas formed 

 by a pair of sutures extending posteriorly from either side of the 

 median lobe to meet the cleft, and extends considerably beyond 

 the posterior edge of the plate. It consists of a short sagittate 

 base and a slender portion divided transversely into two im- 

 bricating sections, each terminating in a short bifid expansion 

 [pi. 24, fig. 5] . In E. f i s c h e r i there is a third part consisting 

 of two, short, flat, diverging crura. As this appears to be a 

 general feature in Eurypterus, it is probable that it exists in 

 this species also. The part of the appendage carried by the 

 second sternite is covered by that of the operculum. It lies in 

 the median cleft which extends through the posterior two 

 thirds of the sternite, the anterior third of the two halves of 

 this plate being fused [pi. 25, fig. 5]. It is slender, being about 

 one fourth as wide as long and does not quite reach the posterior 

 edge of the sternite. The anterior end is slightly constricted 

 where it fuses with the sternite, and the distal is taper- 

 ing. The male appendage is confined to the operculum. 

 In the material of the collection is a single specimen showing 

 [pi. 25, fig. 2] the exterior, the others showing the internal 

 form only. It was evidently small, about one fifth the length 

 of the operculum by which it is surrounded. 



The body is covered with comparatively coarse, imbricating, 

 crescentic scales, most distinct on the sternites and swimming 

 arms. When the integument of the metastoma and paired ap- 

 pendages are scaled away, there remains a punctate surface. 

 The specimens found show that the size of these animals aver- 

 ages from 20 cm to 30 cm. A fragment of the third joint of 

 a swimming arm was found, however, which appears to have 

 been part of an individual over 60 cm in length. 



Genus pterygotus Agassiz. 1839 

 Pterygotus monroensis sp. nov. 



Plate 24, fig. 7, 9 



This species is founded on a single specimen, a cephalo- 

 (horacic shield. The outline of this shield is semielliptic with 



