CEPHALOPODA. 
3G9 
Cyrtoceras eugenium. 
PLATES XXXVI, FIG. 5 ; XLVII, FIGS. 5-7 ; XCVI, FIGS. I-1I; XCVII, FIGS. 10, 11. 
Cyrtoceras eugenium, Halt,. Descriptions of New Species of Fossils, etc., p. 42. 1861. 
“ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 70, pi. 9, figs. 1-3. 1862. 
“ “ “ Illustrations of Devonian Fossils: Cephalopoda, pi. 46, figs. 5-7. 1876. 
Orihoceras foliatum , “ Descriptions of New Species of Fossils, etc., p. 42. 1861. 
“ “ “ Fifteenth Rep. N. Y. State'Cab. Nat. Hist., p. 70, pi. 9, figs. 1-3. 1862. 
Shell slender, apical portion curved, making about one-fourth of one volution. 
Tube nearly straight for a length, measured from the aperture toward 
the apex, equal to one-half the entire length of the shell. In the larger 
individuals the length of the straight portion is comparatively greater. 
Transverse section subcircular, somewhat flattened on the dorsal and ventral 
sides along the straight portion of the tube. Tube regularly enlarging 
from the apex to the aperture, becoming slightly gibbous over the anterior 
half, and contracting very gradually toward the aperture, where it is some¬ 
what expanded. Apical angle about 10°. 
Chamber of habitation with a length one-half greater than the ventro¬ 
dorsal diameter of the tube at the last septum; slightly contracting toward 
the aperture, where it more rapidly expands. Crenulations marking the 
cast of the tube throughout its length, showing as irregular, indistinct, 
rounded furrows and ridges. Aperture entire, with a deep rounded sinus in 
the ventral margin, corresponding to the sinus in the ornaments of the 
test. 
Air-chambers regular, having a depth of six mm. near the grand chamber. 
Toward the apex they are shallower, and show a considerable variation in 
depth; one specimen presents a variation of from four to six mm. in the 
length of twenty mm. Septa smooth, thin, having a concavity greater 
than the depth of the air-chambers, apparently independent of the surface 
characters. Sutures straight and horizontal, but slightly impressed on the 
internal mould. 
Siphunele small, submarginal, exposed in the process of weathering, hav¬ 
ing a diameter of three mm. where the tube measures thirty mm. 
47 
