FOSSILS OF THE NIAGARA GROUP. 147 



CEPHALOPODA. 



Genus ORTHOCERAS, Breyn. 

 Orthoceras annulatum. 



Plate 9, fig. 1. 



Orthoceras annidaturn, Sow.; M. C, tab. 133, 1818. 



0rtli0ce7'atitei< undulatns, Hisinger; Anteckn. V., tab. 4, fig. 0; Vet Akad. Handlin- 



gar, tab. 7, fig. 8. 

 Orthoceratites undulatus ; Lethea Suecica, p. 28, tab. 10, fig. 2, 1827. 

 Orthoceras annulntum ; Miirch. Silurian Syst. and Siluria. 

 Orthoceras undulatum; Pal. N. Y., Vol. II., p. 293, pis. 64 and 65. 

 Orthoceras annulatum, Hall ; 20th Rept- State Cab., p. 351. 

 Orthoceras nodocostatum, McChcsney; New Pal. Foss., p. 94. 

 Orthoceras nodocostatum ; Trans. Chicago Acad. Sci., Vol. I., p. 53, pi. 9, fig. 5. 

 Orthoceras Laphnmi, McCbesney; New Pal. Foss , p. 91. 



Specimens of this species appear to be not uncommon at several local- 

 ities of the Niagara group in Ohio, and frequently present the features 

 of the species in a very good state of preservation. The individuals 

 which have been received for examination range from a diameter of one 

 inch and three-eighths to over two inches and a half. The shell is very 

 gradually expanding from below upward, appearing in examples of a 

 few inches in length to be of nearly or equal size at the opposite ends, 

 very broadly oval in a transverse section, and strongly annulated by con- 

 centric rings, which are more advanced on the sides of the shell in the 

 direction of the longest diameter than on those of the opposite or shorter 

 diameter. Annulations sharply elevated, and rounded on the top, where 

 the substance of the shell is preserved, with broad, deep, concave de- 

 pressions between them ; but where the specimens consist entirely of 

 internal casts of the shell the rings and depressions are much less dis- 

 tinct. Tlie distance of the annulations from each other varies somewhat 

 with the size of the shell, but is not uniform in the different individuals. 

 On a fragment of an inch and a half in diameter the annulations are 

 three-eighths of an inch from crest to crest, giving eight ridges and 

 eight spaces in a length of three inches; and on another individual, 

 where the diameter is two and a half inches, the same number of rings 

 measure three and a quarter inches only, the difference between the two 

 being scarcely perceptible. Septa deeply and regularly concave, their 

 edges corresponding to the external annulations in distance, the ridge of 

 the annulation being placed at or a little in advance of the middle of the 



