394 
PALEONTOLOGY OF NEW YORK. 
Trochoceras discoideum. 
PLATE LIX, FIGS. 6-8. 
Trochoceras discoideum , Hall. Descriptions of New Species of Fossils, etc., p. 36. 1861. 
“ “ “ Fifteenth Rep. N. Y. State Cab Nat. Hist., p. 64. 1862. 
Not “ “ “ Illustrations of Devonian Fossils: Cephalopoda, pi. 59, figs. 1, 2. 1876. 
Trochoceras Clio, Hall. Fifteenth Rep. N. Y. State Cab. Nat. Hist., pi. 9, fig. 8. 1862. 
“ “ Hall (in part). Illustrations of Devonian Fossils : Cephalopoda, pi. 59, figs. 3-7. 1876. 
Shell discoiclal, 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 strise, 
crossed by regular, more distant, sharp, revolving striae, forming a reticula¬ 
tion of the surface. Tube ornamented with rather prominent, transverse, 
linear nodes or annulations 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. 
