286 PALEONTOLOGY OF NEW YORK. 



septa having been destroyed, and the siphuncle falling down against the 

 interior surface. Its diameter is 1.5 mm. where the tube has a diameter of 

 t«n nun. 



Test thin. Surface marked by fine, sharp, longitudinal stria3, crossed by 

 concentric striae, forming a regular reticulation of the surface ornamentation. 



The specimen has a length of eighty-five mm., and a diameter at the 

 larger extremity of twenty-four mm. 



Tbis species differs from O. tenere, with which it is associated, in the decided 

 reticulation of the surface stria?, which, in that species, are curving lines of 

 growth, without longitudinal markings. 



This form, of which but a single specimen has been observed, occurs in a 

 completely silicified condition in the compact cherty limestone at the Falls of 

 the Ohio. The surface-markings are very perfectly preserved, and are charac- 

 teristic of the species. 



Formation and locality. In the upper limestone layers belonging to the Ham- 

 ilton group, at the Falls of the Ohio, near Louisville, Ky. 



Orthoceras tersum, n. sp. 



PLATE LXXXIV, FIG. 5. 



Shell slender, straight, regularly and gradually enlarging from the apex. 

 Transverse section subcircular. Apical angle about 7°. Initial extremity 

 unknown. 



Chamber of habitation not observed. Air-chambers regular, very gradu- 

 ally increasing in depth toward the outer chamber, varying from two to 

 three mm. in the length of thirty-five mm. The ventral side is indicated on 

 the internal mould by a low, longitudinal, continuous carina. 



Septa smooth, so far as observed, with the exception of an areola around 

 the insertion of the siphuncle. Sutures straight and horizontal. 



Siphuncle small, subcentral, nearer to the dorsal side, having a diameter of 

 less than one mm. at the septa, where the diameter of the tube is about ten 

 mm. Its elements in the interseptal spaces have not been observed. 



