CEPHALOPODA. 
293 
Orthoceras aulax, n. sp. 
PLATE LXXXIV, FIG. IS. 
This species is indicated from a fragment of the chamber of habitation and 
several of the attached air-chambers, which retains characteristic and distinc¬ 
tive surface-markings. The tube is slender, and the enlargement very 
gradual. Air-chambers, septa and siphuncle unknown. Surface marked by 
regular, numerous, low, rounded transverse ridges, about twelve in the space 
of ten mm. The interspaces, or furrows, are regularly concave, and have a 
width equal to the ridges. Portions of the surface also show that it was 
traversed by fine, sharp, longitudinal strim, crossing the ridges. Length of the 
fragment, eighty mm. 
The specimen is from the soft shales, and is much compressed. It occurs 
with 0. emaceratum , from which it is distinguished by its surface ornamentation. 
Formation and locality. From the shales of the Hamilton group at Hamburg, 
Erie county, N. Y. 
Okthoceras scintilla, n. sp. 
PLATES LXXXIV, FIGS. 19-21; CXIII, FIGS. 6-12. 
Shell minute, slender, regularly and gradually enlarging from the apex. 
Transverse section circular. Apical angle about 3°. Initial extremity 
unknown. 
Chamber of habitation not observed. Air-chambers increasing in depth 
from the apex toward the outer chamber; having, at that point, a depth 
equal to the diameter of the tube, or from one to two mm. 
Septa thin, moderately concave, smooth, with the exception of a large cir¬ 
cular areola around the insertion of the siphuncle. Sutures straight and 
horizontal. The areola is always well defined, constituting the base of the 
cone around the siphuncle in the anterior portion of each air-chamber. 
Siphuncle large, central, cylindrical, indenting the septa, having a diameter 
of about one-third the diameter of the tube. Toward the outer chamber 
