268 
PALAEONTOLOGY OF NEW YORK. 
mm., crossed bj fine, irregular, lamellose lines of growth. The internal 
mould retains the impression of the strong longitudinal, surface striae. 
A specimen, consisting of a portion of the chamber of habitation, and 
about seventeen of the preceding air-chambers, has a length of 100 mm., and 
is imperfect at both extremities. 
This species is separated from 0. jaculum on account of its shallower air- 
chambers, more excentric siphuncle, and longitudinal surface striae. 
Formation and locality. In limestone of the Upper Ilelderberg group, from 
Western New York. 
Orthoceras rudicula. 
PLATES XXXVII, FIG. 2; CXII, FIGS. 1,2. 
Orthoceras rudicula, Hall. Illustrations of Devonian Fossils : Cephalopoda. Explanation of plate 37, 
fig\ 7. 1876. 
Shell straight, rapidly enlarging. Transverse section subcircular. Apical 
angle about 11°. Initial extremity unknown. 
Chamber of habitation not observed. Air-chambers increasing in depth 
from the apex to the outer chamber, and varying from three to seven mm. 
Septa with a concavity equal to an arc of 115°. There is a large, elevated 
areola around the insertion of the siphuncle, which becomes more prominent 
toward the apex, and is surrounded and covered by a minute mammillary 
deposit of organic matter. 
Siphuncle subcentra], expanding between the septa; the walls are very 
thin, and are obliterated or absorbed in the majority of the air-chambers. 
Its diameter at the septa is about two mm. where the tube has a diameter 
of twenty-four mm. 
Test and surface ornamentation unknown. 
The specimen embraces about twenty air-chambers, and has a length of 
100 mm. 
The air-chambers have been separated and displaced by compression, and 
the external walls in the cast have become concave. The appearances of 
