no SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 75 



for S. curekensis (Walcott) and the latter for 6'. illaenoides (Bil- 

 lings). Indeed, the cranidium in the new genus is considerably like 

 that of such typical species of Symphysurns as S. angustatus and 

 S. palpebrosus. Carefully compared, however, certain differences 

 appear : the eyes are larger and situated nearer the front in Sym- 

 physurus; the dorsal furrows are deeper and in front of the eyes 

 follow the suture, there being in this part no fixed cheek. In Sym- 

 physvtrina, on the contrary, there is always — even though the dorsal 

 furrow is shallow — some space left on the antero-lateral parts of the 

 cranidium that may be called a fixed cheek. As for the posterior 

 fixed cheeks, these are relatively larger in Syinphysurns, while the 

 opposite is true of the free cheeks. The free cheeks show another, 

 probably more important, difference in the extension of the doublure 

 beneath the front of the cranidium. As described by Brogger and 

 others these extensions from the opposite cheeks unite in the middle 

 of the head of typical species of Symphysunis and Nileus without 

 leaving a suture where they join. The two cheeks thus are united 

 into a single piece. In Syinphysurina, however, this never occurs, 

 the two cheeks being separated by a suture precisely like that found 

 in all of the true Asaphidae. 



" Oo the other hand, as already indicated, the cephalon of ^^3;^^- 

 physurina is not greatly different from that of Nileus and Sym- 

 physunis. However, only the latter requires close comparison because 

 Nileus is distinguished readily enough by its evenly convex, unlobed 

 and unsegmented pygidium, very wide thoracic axis, and larger eyes. 



" Regarding Symphysurus so far as shown by the collections in 

 the U. S. National Museum, this genus is not represented in American 

 deposits. Raymond ^ has on two occasions referred American species 

 to it. They do not appear to belong to Symphysurus but to Walcott's 

 Tsinania. The name of the New Jersey species, therefore, becomes 

 Tsinania columhicnsis (Weller), while that from the Valley of New 

 York becomes T. convexus (Cleland), providing Whitfield's Illae- 

 nurus convexus is not congeneric. 



" The reference of Illaenurus eurekensis Walcott ^ to Symphysurus 

 by Raymond ^ had been anticipated by Brogger's * suggestion. Ray- 

 mond in the same paper refers also Asaphus illaenoides Billings to 



^Annals Carnegie Mus., Vol. 7, 1910, pp. 42-44. Roy. Soc. Canada, Proc. and 

 Trans., 3d ser., Vol. 5, 1912, sec. 4, p. 117. 



^ Paleontology of the Eureka District : Monogr. U. S. Geo!. Surv., Vol. 8, 

 1884, P- 97, pl- 12, figs. 4, 4a. 



' Annals Carnegie Mus., Vol. 7, 1910, p. 144. 



* Ueber die Verbreit. der Euloma-Niobe Fauna : Nyt. Mag. for Naturvidensk., 

 Vol. 36, 1898, pp. 189, 228, Christiania. 



