488 Systematic Paleontology 



somewhat abruptly elevated concentric annulations, at least five occurring 

 in the space of 10 mm. Surface too badly weathered to observe any trans- 

 verse or longitudinal striae if present. 



This species closely resembles Orthoceras tenuiannidaium (Hall) of 

 the Helderberg. It may be distinguished, however, by its smaller apical 

 angle and by possessing shallower and more numerous annulations. 



Occurrence. — Helderberg Formation, Keyser Member. Cumber- 

 land. 



Collection. — U. S. National Museum. 



[Maynard.] 



Orthoceras rigidum Hall? 



Plate LXXXVIII, Fig. 5 



Orthoceras rigidium Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 344, pi. Ixx, 

 figs. 3a-d, 1861. 



Description. — "Shell elongate, gradually tapering; section circular. 

 Septa moderately convex, distant from each other about one-sixth the 

 diameter of the tube. Siphuncle central, narrow in its passage through 

 the septum. Surface marked by fine sharply elevated equal transverse 

 striae." Hall, 1859. 



Occurrence. — Helderberg Formation, Keyser Member. Devil's 

 Backbone. 



Collection. — Maryland Geological Survey. 



[Maynard.] 



Suborder CYRTOCHOANITES 



Family OOCERATIDAE 



Genus CYRTOCERAS Goldfuss 

 Cyrtoceras ? dubium n. sp. 

 Plate LXXXVIII, Fig. 

 Description. — Shell small, arcuate, tapering. Chamber of habitation 

 about one-third total length, inflated, slightly constricted towards aper- 

 ture. Air chambers subcqual in depth, diminishing rapidly in diameter 



