REPORT OF THE DIRECTOR I918 209 



legs, through its series of fine, long, slender spines on the fourth 

 joint, appeared to us as a typical representative of the subgenus 

 Ctenopterus of Stylonurus, no such longitudinal series having then 

 been observed in any other Eurypterid genus except Ctenopterus 

 and the Ordovician Echinognathus. In the latter of these, how- 

 ever, the spines are differently shaped and form a contiguous series, 

 and the endognathite is not slender but short and compact. 



To our surprise, the collecting in the Vernon shale furnished speci- 

 mens that combine this Ctenopterus leg with the body of an Eusarcus, 

 as far as outline of carapace, position of e^^es, form of metastoma, 

 predominant length of second pair of endognathites, shape of swim- 

 ming legs, surface sculpture and curved telson spine are concerned. 

 On the other hand, the preabdomen is not much broader than the 

 posterior edge of the carapace and not at all broadly subcircular, as 

 in Eusarcus; and gradually contracting into the postabdomen, in 

 this regard again recalling Stylonurus. It is thus obvious that this 

 form combines, in a curious way, characteristic features of Eusarcus 

 with certain characters of Stylonurus. 



A character apparently foreign to both Stylonurus and Eusarcus 

 is the development of two longitudinal furrows on the tergites of the 

 postabdomen, which separate the flat lateral portions of the tergites 

 from the convex axial part and give the abdomen a trilobed aspect. 

 This feature had already been observed by F. Schmidt (1904) in 

 his Stylonurus simonsoni and described as " Dorsalfur- 

 chen." Clarke and Ruedemann (1912, p. 231) have later placed this 

 species of the Baltic Silurian with Eusarcus and it is qmte probable 

 that it is congeneric with our multispinosus. 



The generic term Mixopterus^ is here proposed for forms which, 

 like the genotype M. multispinosus, unite the following 

 characters of Eusarcus, namely: subtriangular carapace; forward 

 submarginal position of compound eyes, subtriangular form of metasr 

 toma, predominant length of second pair of endognathites and 

 decreasing length of following endognathites, powerful swimming 

 legs with long blade and short proximal joints, and curved telson 

 spine, with the following characters of Stylonurus (subg. i, Ctenop- 

 terus), namely: a relatively narrow preabdomen, and gradually 

 contracting postabdomen, and a series of longitudinally arranged 

 vertically attached spines upon the longer endognathites. A feature 

 sui generis in this genus are the " dorsal furrows " upon the preab- 

 domen. 



^From Greek MZj^f, a mingling. 



