CEPHALOPODA. 233 



ORTHOCERATA OF THE SCHOHARIE GRIT. 



DESCRIPTIONS OP SPECIES. 



Section a. — Robust forms, showing a gradation from a simple areola on the septum surrounding 

 the siphuncle, to a decided and complex organic deposit. 



Orthoceras Pelops. 



PLATES XXXV, FIGS. 1-3; XXXV A, FIGS. 1-6; XXXVII, FIGS. 3,4; LXXVIII B, FIG. 2. 



Orthoceras Ptlirps, Hall. Descriptions of New Species of Fossils, etc., p. 45. 1861. 

 Fifteenth Kep. N. V. State Cub. Nat. Hist., p. 73. }862. 

 " ■' " in part- Illustrations of Devonian Fossils : Cephalopoda, pi. 35. 1S76. 



Shell robust, straight, elongate. Transverse section circular in specimens not 

 compressed ; the flattening often amounts to a change in the relations of the 

 diameters of from 1-1 to 1-3. Cone gradually and regularly enlarging 

 from the apex. Apical angle 7° to 9£°. Initial extremity unknown. 



Chamber of habitation well developed, sometimes measuring more than 

 twenty-five centimetres in length, and at least five or six times as long as 

 the diameter at the last septum ; a broad, gentle constriction near the aper- 

 ture is generally present. Depth of the air-chambers about one-sixth of the 

 width of the tube ; four to eight in the space of sixty mm., depending on 

 the diameter of the tube or the distance from the apex. 



Septa thin, with a small areola around the insertion of the siphuncle, regu- 

 larly concave; concavity equal to an arc of from 120° to 126^°; part of this 

 variation being probably due to compression, or other influences which have 

 affected the true curvature. In the more perfect specimens, the sutures are 

 horizontal and straight, the various degrees of distortion observed having in 

 many cases made them curved and oblique to the axis. 



Siphuncle central, or very slightly excentric ; walls thin, seldom pre- 

 served ; width equal to about two-thirds the depth of the adjacent chambers, 

 or approximately from four to five mm. The presence of an areola on the 

 septa indicates that the siphuncle was moniliform, or slightly expanding. 

 30 



