CEPHALOPODA. 253 



A fragment preserving eight air-chambers measures eighty-eight mm. in 

 length, and has a diameter of forty-five mm. One specimen retaining the 

 chamber of habitation and five ordinary chambers, has a length of 125 mm., 

 with a diameter near the aperture of thirty-five mm. 



This species closely resembles 0. tetricum, but is readily distinguished by its 

 more regular and frequent septa, their lesser concavity, and the decided con- 

 striction of the chamber of habitation, 



Formation and localities. In the Schoharie grit at Schoharie, and in the 

 Helderberg mountains. 



Orthoceras stylus. 



PLATES XXXVI, rUM. 2,3; and LXXIX, FIGS. 1,3. 



Orthocera* baculum. Hall. Descriptions of New Species of Fossils, etc., p. 46. 18(51. 



Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 73. 1862. 

 " " Illustrations of Devonian Fossils: Cephalopoda. Explanation of plate 36 



figs. 3, 4. 1876. 

 Not " " Meek. Proc. Acad. Nat. Bel., Pliila. 1860. 



X ' " baculux. Barhasdk. Syst. Sil. du centre de la Boheme, vol. 11, text 3, p. 118. 1874. 



" x/i/Ihx, Hall. Catalogue of American Palaeozoic Fossils (S. A. Miller), p. 245. 1877. 



Smell an extremely slender, straight, cylindro-conical tube. Transverse sec- 

 tion circular. Apical angle 2°. Apex unknown. 



Chamber of habitation long, cylindrical, length more than six times the 

 transverse diameter, showing no variation from the general contour of the 

 tube. Aperture not observed. Air-chambers regular, shallow, the depth 

 being about five mm. Usually the last two are shallower than the posterior 

 chambers. 



Septa thin, ornamented with a small, circular areola around the insertion 

 of the sipliuncle; concavity equal to an arc of from 115° to 120°. Sutures 

 straight and horizontal. 



Siplmncle small, subcentral or slightly excentric, diameter 1.5 mm. at the 

 septa, where the diameter of the tube is fifteen mm. Its characters have 

 not been observed in its passage through the chambers. 



The test has not been preserved, but the surface characters are sometimes 



