148 PALEONTOLOGY OF OHIO. 



chamber, and the edges of the septa, in the direction of the longest 

 diameter of the shell, being as much in advance of those on the flattened 

 sides as will correspond with the increased diameter of the tube in that 

 direction. 



Surface of the shell marked by closely arranged, concentric lines of 

 growth, which are strongly undulated and lamello'se, but less distinctly 

 marked on the ridges than between. There are also distant parallel 

 ridges traversing the shell longitudinally, and forming slight nodes 

 where they cross the concentric annulations. The surface characters, 

 especially the longitudinal ridges, are often quite obscure, and where the 

 specimens are casts of the interior, the lamellose lines are seldom, if 

 ever, seen, although the. nodes left by the ridges, where they cross the 

 annulations, are in these cases frequently quite distinct. 



The species has been so frequently described, and is so universally 

 known, that it is not necessary to institute comparisons between the sev- 

 eral forms which have given rise to the many synonyms under which it 

 has appeared. 



Formation and locality : In the limestones of the Niagara group, at Yellow Springs, 

 Cedarville, and elsewhere in Ohio. 



Orthoceeas ceebescens. 



Plate 9, fig. 2. 

 Orthoceras crebescem, Hall; 20th Eept. State Cab., p. 354, pi. 19, flgs. 1-3. 



Shell of rather large size, gradually and moderately tapering ; trans- 

 verse section circular, or sub-circular, from compression; septa deeply 

 and evenly concave, quite regularly disposed and distant, about three 

 and a half of the chambers occupying a length equal to the diameter of 

 the sheil where counted ; siphuncle very large, and centrally situated ; 

 surface unknown. 



The examples, being all internal casts, do not preserve the features of 

 the exterior surface, but that of the casts is perfectly smooth, so far as 

 can be determined. There are some points of difference between the 

 Ohio specimens and those from the more western localities froni which 

 the original description of the species was taken, as the slightly greater 

 distance of the septa, and the more distinctly oval form of the shell. 

 This latter feature, however, may have been caused to considerable ex- 

 tent by compression, as all of the individuals observed have the flatten- 

 ing in the direction of the plane of the strata, as far as can be deter- 



