28 PPP, KEroUT OF PKOGUKSS. JAMES HALL. 



metastomn or post-oral plate found both in the same associ- 

 atit)n anil attached is of similar form, with a proportionally- 

 greater width than in the ordinary forms of Eurypterus, 

 and api)roaches more nearly to the form of the same ap- 

 pendage in Pterygotus. The single dactylus at the extrem- 

 ity of the palpi corresponds with known forms of Eurypte- 

 rus described by Mr. Henry Woodward, and may also be com- 

 pared with the corresponding part of Pterygotus. The spini- 

 form extensions at the post-lateral extremities of the seg- 

 ments of the body and thorax are but a more extreme devel- 

 opment of a feature which is common to all true Eurypte- 

 rides, and can scarcely be considered of generic importance. 



In the elongated joints of the swimming foot and their 

 serrated margins the Pennsylvania forms resemble Doli- 

 chopterus, and may be referred to that sub-genus with as 

 much propriety as to any other sub-generic form. In this 

 respect the Eurypterus {Antliraconectes) Mazonensls is still 

 more similar to Dolichopterus in its simple elongate median 

 appendage of the thoracic plate. The two small accessory 

 lateral plates of the median appendage of that species have 

 not been detected in E. Mansfieldi, and their presence in 

 the former species may perhaps, if verified, be considered as 

 of sufficient importance for the separation of that form as a 

 sub-genus. Finally, the forms are not as far removed from 

 the typical species of Eurypterus, in any of their charac- 

 ters, as are several of those which Mr. Woodward has de- 

 scribed from the Upper Ludlow rocks of England, and which 

 he does not hesitate to place under the genus Eurypterus. 



The Pennsylvania forms are quite unlike the European 

 carboniferous species in their general aspect and j^ropor- 

 tions, as well as in the details of parts of their organization, 

 and in these respects more nearly resemble the tyjiical forms 

 or the genus. 



In addition to the two very well-marked forms described, 

 there are, in the collections examined, several fragments of 

 other crustaceans of this family which cannot be satisfac- 

 torily referred to genus and species. Two of these are fig- 

 ured on Plate IV, Figs. 9, 10. The specimen, Fig. 9, is an 

 ectognath belonging to a large form of which we have also 



