﻿Cephalopoda of the Cincinnati Group. 237 



Genus i. ORTHOCERAS, Breynius. 1732. 



De melonibus petrefactis montis Carmel. Miller, 1875, Cin. Quar. Jour. 



Sci., II, 124. 



Shell conical, straight ; greater part of posterior end traversed by- 

 convex, transverse septa ; transverse section circular, oval or more or less 

 triangular; siphuncle cylindrical, or dilated between the chambers, vary- 

 ing in position from the centre to the outer margin ; surface smooth, or 

 longitudinally, or transversely lined. 



KEY TO SPECIES. 



Septa distant ; siphuncle eccentric. O. amplicameratum, 1. 



Septa approximate ; siphuncle varying in position. O. dyeri, 2. 



Siphuncle central; surface smooth. O. mohri, 3. 



Surface finely striated transversely. 



O. junccum, 4. 



Siphuncle eccentric ; surface with transverse lines. 



O. transversum, 5. 



Siphuncle lateral; surface with obscure, longitudinal ridges. 



O. ortoni, 6. 



Surface with profoundly wrinkled longitudinal lines. 



O. anellum, 7. 



Siphuncle sub-central ; septa annulated, concave. 



O. annulatum, 8. 



Surface with low, rounded, longitudinal ridges. O. turbidum, 9. 



Siphuncle cylindrical; shell semi-cylindrical. O. hindci, 10. 



Edges of septa raised ; narrow, raised, longitudinal line along one 

 side of shell. O. duseri, 11. 



Shell surrounded by two tubes ; outer tubes without septa. 



O. carleyi, 12. 



Siphuncle expanded into bladder-like rings at septa ; surface marked 

 by undulated, thread-like, longitudinal lines. O. tenuifilum, 13. 



I. O. AMPLICAMERATUM, Hall. 1 847. 



(Pal. N. Y. I, 205. PI. 51, figs. 1 a-g.) 



Teretely cylindrical, extremely elongated, very gradually tapering; 

 septa very convex, distant ; siphuncle eccentric, small ; section circular ; sur- 

 face unknown. 



Locality : Cincinnati, Lebanon, O. 



The septa in this species are quite large and distant. It is easily 

 recognized by this character and by the very gradual tapering. 



