278 PALiEONTOLOGY. 



CEPHALOPODA. 



Genus CYRTOCERAS Goldf. 

 Cyetoceras cessa'ior n. sp. 



Plate VI, (ig. 15. 



Shell of rather small size, moderately curving throughout its length, 

 and rapidly expanding from below upward ; the specimen measured show- 

 ing an increase of diameter from less than half an inch to about eight-tenths 

 of an inch in a length of only about nine-tenths of one inch ; section circu- 

 lar. Surface marked by strong, rather distant, rounded annulations, which 

 are separated by concave interspaces. The annulations are directed slightly 

 upward or forward in crossing the back of the shell, and become gradually 

 more distant with the increased growth of the individual ; four of these 

 annulations occupy a length of the shell equal to its diameter at the upper- 

 most of those counted. Septa equal in number to the annulations, their 

 extreme outer margins reaching nearly to the crest of the ridges in some 

 cases ; others are more distant. Siphuncle small, submarginal, situated a 

 little to the right of the dorsal line (perhaps only an accidental feature). 



Surface of the shell marked by fine, crowded, thread-like, encircling 

 striae on both ridges and interspaces. 



The species closely resembles in many of its characters that figured 

 by Meek and Worthen (Geol. Ills., vol. ii, plate 24, fig. 3), under the name 

 of Orthoceras annuJato-costatum, but differs in its circular section, more rapidly 

 expanding tube, and longitudinal curvature. The latter feature, together 

 with the dorsally-situated siphuncle, would place the species under the 

 genus Cyrtoceras, and we strongly suspect the Illinois shell will also prove 

 to belong to the same genus when its true characters are ascertained. The 

 0. Chesterensis of Swallow (Trans. St. Louis Acad. Nat. Sci., vol. ii, p. 98) 

 is still further removed from this one by its closely-amvnged annulations, 

 though it is not stated if it be curved or straight. 



Formation and locality. — In black shales of probably Coal-Measure age, 

 near Eberhardt Mill, White Pine, Nevada; associated with Goniatltes Kingii 

 and Cardiomorpha M'mouriensis. Collected by Arnold Hague, esq. 



