CEPHALOPODA. 261 



Section e. — Annulated forms. 

 Orthoceras Thoas. 



PLATES XLI, FIGS. 1-9; LXXVIII B, FIG. 5; LXXIX, FIG. IS; 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. 



" Hyas, " Descriptions of New Species of Fossils, etc., p. 47. 1861. 



" " 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. 1876. 

 Compare Huronia Stokesi, Castblkau. Essai sur le Syst. Sil. de l'Amcrique Septentrionale, p. 33, pi. 9, 



fig. 2. Paris, 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 depth of about five 

 mm. near the chamber of habitation. 



Septa smooth, slightly indented by the siphuncle, having a concavity 

 equal to an arc of about 140°. Suture lines straight and horizontal. 



Siphuncle 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, 



