393 PAUEOSTOLOQV OF NEW YORK. 



Trociioceras Clio. 



PLATES LIX, FIGS. 1, t, S, 4, 5, 9; CXt, FIG. 6. 



Trochocerai Clin, Hill. Fourteenth Rep. N. Y. State Cab. Nat. Hist., p. 108. 1861. 

 Not « " " Fifteenth Rep. N. Y. State Cab Nat. Hist., pi. 9, fig. 8. 1802. 



Not " " Hall (in part). Illustrations of Devonian Fossils: Cephalopoda, pi. 59, figs. 3-7. 1876. 



TYocIhkvtv* disctrideuni. Hall. Illustrations of Devonian Fossils: Cephalopoda, pi. 59, figs. 1, 2. 1876. 



Shell subdiscoidal, sinistral, making about three volutions, coiled in a helix. 

 Volutions contiguous, supporting each other on the upper dorso-lateral side, 

 which is marked by a revolving furrow, made by the inner, adjacent volution. 

 The spire has an elevation equal to about two-thirds the transverse diameter 

 of the disc. Umbilicus profound, having a diameter of about one-half the 

 diameter of the tube. Transverse section very broadly subovate, lateral 

 diameter the longer. The plane of the lateral diameter is oblique to the 

 vertical axis of the helix, making an angle of nearly 45°, and is directed 

 away from the apex of the spire, while the plane of the ventro-dorsal diameter 

 is inclined to the apex. Tube regularly and gradually enlarging, increasing 

 its diameter about twice for each volution. 



Chamber of habitation large, having a length more than twice the diameter 

 of the tube at the last septum, following the curvature of the spire. It 

 comprises about one-third of the outer volution, and has a capacity of less 

 than one-half of the volume of the air-chambers. Aperture slightly con- 

 tracted, opening downward, oblique to the spiral axis of the tube, and 

 showing a sinus in the ventral margin corresponding to the sinus in the 

 stria) and annulations of the test. 



Air-chamhers regular, increasing in depth toward the grand chamber, 

 where they have a depth of six mm. in an individual having a diameter of 

 forty-five mm., measured across the disc. Septa smooth, not regularly 

 concave. The concavity is greater in a ventro-dorsal direction, and is about 

 equal to the depth of one air-chamber or an arc of 110°. Sutures straight 

 and at right angles to the spiral axis of the tube, crossing the annulations, 

 which are about twice as frequent. 



