CEPHALOPODA. 
261 
Section e.-—Annulated forms. 
Ortiioceras Thoas. 
PLATES XLI, PIGS. 1-9; LXXVIII B, FIG. 5; LXXIX, FIG. 13; LXXX, FIGS. 7, 10, 11 ; CXII, FIGS. 7,8. 
Orthoceras Thoas, Hall. Descriptions of New Species of Fossils, etc., p. 47. 1861. 
“ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 75, pi. 7, fig. 4. 1862. 
“ Hi/as, “ Descriptions of New Species of Fossils, etc., p. 47. 1S61. 
“ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 75, pi. 7, fig. 5. 1862. 
“ Thoas, “ Illustrations of Devonian Fossils : Cephalopoda. Explanation of pi. 41. 1S76. 
Compare Haronia Stokesi, Castelnau. Essai sur le Syst. Sil. de l’Amerique Septentrionale, p. 33, pi. 9, 
fig. 2. Pai is, 1843. 
Shell straight, very gradually enlarging from the apex to the aperture. Trans¬ 
verse section circular. Apical angle from one to two degrees. Initial point 
not observed. 
Chamber of habitation well developed, cylindrical, without any marked 
constriction or expansion near the aperture; length more than three times 
the diameter at its base. Aperture unknown. Air-chambers regular, cor¬ 
responding in number with the annulations, and having a dejith of about five 
mm. near the chamber of habitation. 
Septa smooth, slightly indented by the sipliuncle, having a concavity 
equal to an arc of about 140°. Suture lines straight and horizontal. 
Sipliuncle central, moniliform, having a diameter at the septa of two mm. 
where the tube measures twenty mm., and expanding in its passage through 
the chambers to a diameter of four mm. 
Test rarely preserved, but the surface-markings are impressed upon the 
internal casts of many of the specimens. Its thickness, as observed from 
specimens in the Upper Helderberg limestone, is .75 of one mm. The 
ornamentation of the tube consists of regular, more or less sharp, horizontal 
annulations, increasing in frequency toward the apex. The furrows between 
the annulations are regularly concave, becoming flat as the distance between 
the annulations increases. These distances vary from three to eleven mm. 
on different portions of the tube. Surface of the test marked by fine, 
regular, continuous, transverse striae, crossed by more distant, sharp, elevated, 
