394 PALEONTOLOGY OF NEW YORK. 



Trochoceras discoideum. 



PI-ATE LIX, FIG8. 8-8. 



Trochocerax ditcoideum. Hall. Descriptions of New Species of Fossils, etc., p. 36. 1861. 



Fifteenth Rep. N. Y. Slate Cab Nat. Hist., p. 64. 18152. 

 PTi>t " " " Illustrations of Devonian Fossils: Cephalopoda, pi. 59, figs. 1, 2. 1876. 



Trot/»*erwt CTio, Hall. Fifteenth Rep. N. Y. State Cab. Nat. Hist., pi. 9, fig. 8. 1802. 



« " Hall (in part). Illustrations of Devonian Fossils : Cephalopoda, pi. 59, figs. 3-7. 1876. 



Shell discoidal, sinistral, making about three contiguous volutions, supporting 

 each other on the upper dorso-lateral side. Spire depressed, sinistral. 

 Umbilicus large, wide, exposing all the volutions. Transverse section sub- 

 circular. Tube regularly and gradually enlarging from the apex, more 

 rapidly so in the outer volution. 



Chamber of habitation large, curving with the volutions of the helix, 

 occupying more than one-third of the outer volution. Aperture expanded. 



Air-chambers regularly increasing in depth toward the grand chamber, 

 equal in frequency to the transverse nodes, varying from four to six mm. on 

 the outer volution of different specimens. These measurements are taken 

 from the lateral face of the tube. Septa smooth, not regularly concave, 

 having a concavity equal to an arc of about 130°. Sutures straight, and at 

 right angles to the spiral axis. 



Siphuncle small, central, surrounded, at its insertion in the septa, by a 

 small areola which extends, as a narrow slightly raised expansion, to the 

 ventral margin. 



Test not preserved. Surface marked by fine, sharp, concentric striae, 

 crossed by regular, more distant, sharp, revolving stria?, forming a reticula- 

 tion of the surface. Tube ornamented with rather prominent, transverse, 

 linear nodes or ann illations on the lateral faces, which are not continued 

 over the dorsal and ventral sides. The nodes are of about the same frequency 

 as the septa, and reach their greatest prominence on the ventro-lateral faces 

 of the tube. Sinus not observed. 



The internal mould shows the linear, lateral nodes somewhat subdued, 

 and is otherwise essentially smooth. Sutures slightly impressed. 



