CEPHALOPODA. 313 



dinal carina along the internal mould of the chamber walls. Sutures straight 

 and horizontal. 



Siphuncle small, subcentral, moniliform, with a diameter at the septa of 

 two mm., where the tube measures thirty mm. 



Test and surface-markings not preserved. The internal mould is smooth, 

 with the exception of the ventral carina along the walls of the air-chambers. 



The specimen figured, and the only one observed, has a length of thirty-five 

 mm., with a diameter of about thirty mm., and preserves four air-chambers. 



This species is distinguished by the remarkable ornamentation of the septa, 

 and has no allied form in this horizon. 0. pravum, from the Schoharie grit, 

 shows an ornamentation on the septa from an organic deposit, or markings on 

 the mantle of the mollusk ; but in this species it is quite different, and the 

 other characters are very unlike. 



Formation and locality. In the semicalcareous beds of the upper member of 

 the Chemung group, in Southern New York. 



Orthoceras bipartitum, n. sp. 



PLATE CXIII, FIGS. 20, B. 



• 



Shell straight, regularly and gradually enlarging to the outer chamber. Trans- 

 verse section subcircular. Apical angle about 5°. Initial extremity unknown. 



A small portion of the chamber of habitation has been observed, which 

 possesses no unusual features beyond the ornamentation of the tube. Air- 

 chambers regular, numerous, increasing in frequency toward the apex, vary- 

 ing in depth from one to four mm. in the length of thirty mm., measured 

 from near the chamber of habitation. 



Septa thin, smooth, slightly concave. Sutures straight and horizontal. 



Siphuncle excentric, with a diameter of 1.5 mm. where the tube measures 

 nine mm., and distant from the nearest septal margin one-fourth of the 

 diameter of the tube. Its elements have only been observed at the septa. 



Test not preserved. The cast, near the apex, is smooth. Toward the 

 chamber of habitation the surface is raised into low, rounded undulations, 

 40 



