274 
PALAEONTOLOGY OF NEW YORK. 
Section a. — Robust forms. 
Orthoceras Eriense. 
PLATES XL, PIGS. 1-4; LXXXVII, FIGS. 1, 2. 
Orthoceras robustuin, Hall. Illustrations of Devonian Fossils’: Cephalopoda. Explanation of pi. 40. 1876. 
Not Orthoceras robustum, Winchell. Am. Jour. Sci.-Arts, 2d series, vol. 33. 186-2. 
Not “ “ Barrande. Syst. Silur. du centre de la Boheme. 1866. 
“ Eriense, Hall. Catalogue Am. Pal. Fossils, S. A. Miller, p. 174. 1877. 
Shell very large, straight, robust, regularly enlarging to the aperture. Trans¬ 
verse section circular. Apical angle 8°. Initial extremity unknown. 
Chamber of habitation well developed, having a length equal to twice its 
transverse diameter through the centre, with a diameter at the last septum 
of three-fourths the diameter at the aperture. The tube regularly expands 
to the aperture, which is slightly contracted. Aperture entire, with a gentle 
retral curve on the ventral margin; margins acute. Air-chambers regular, 
numerous, having a depth of from ten to fifteen mm., gradually becoming 
more numerous toward the apex. The internal cast of the walls is smooth 
and flat, with a minutely longitudinal, striate band around the anterior mar¬ 
gin of each air-chamber, with more irregular, slightly undulating, concentric 
strife over the central portion 
Septa thin, smooth, having a concavity equal to an arc of 116°, or a depth 
greater than that of the chambers. Sutures straight and horizontal. 
Siphuncle large, straight, cylindrical, with a diameter of eight mm. where 
the tube has a diameter of seventy-five mm. 
The test has a thickness of nearly one mm. on the chamber of habitation. 
Surface cancellated; marked by regular strife of growth, interrupted by fine 
longitudinal strife. 
The internal mould is essentially smooth, with the septal sutures but 
slightly indented. The very fine, longitudinal and transverse strife over a 
portion of the chamber walls are characteristic but not prominent. 
A specimen preserving an apparently entire chamber of habitation, and 
two of the adjacent air-chambers, has a length of twenty-one centimetres, 
