CEPHALOPODA. 295 
two mm. where the tube has a diameter of twenty-five mm. In the cavities 
of the air-chambers its elements have not been observed. 
The test has a thickness of nearly one mm. over the chamber of habita¬ 
tion. Surface marked by regular longitudinal ridges, with a single interme¬ 
diate smaller ridge between, crossed by fine, sharp, crenulating, concentric 
striae. 
The internal mould usually preserves the longitudinal furrows and ridges 
without the concentric striae, but they are less prominent than on the exterior 
surface of the test. 
One fragment, consisting of the chamber of habitation and several attached 
air-chambers, has a length of 135 mm., with a diameter of nearly fifty mm. 
at the larger extremity. Other smaller fragments have been observed 
which do not furnish any additional data as to the dimensions of the shell. 
This species is distinguished from 0. profundum of the Upper Helderberg 
group, by the finer concentric strife and the intermediate smaller longitudinal 
ridges between the prominent ones; and from 0. JEgea by the absence of the 
low annulations of the tube and the numerous and irregular longitudinal ridges 
and striae of that species. 
Formation and localities. In the shales of the Hamilton group, at York, Gen- 
eseo, Avon and other localities in the western portion of the State. 
Orthoceras jEgea. 
PLATE LXXXII, FIGS. 7-13. 
Orthoceras JEyea. Hall. Descriptions of New Species of Fossils, etc., p. 52. 1861. 
“ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 80. 1862. 
Shell straight, regularly and somewhat rapidly enlarging. Transverse section 
very broadly elliptical. Apical angle about 10°. Initial extremity unknown. 
The extent of the chamber of habitation has not been observed. Air- 
chambers regular, having a depth of about seven mm. 
Septa smooth, with a concavity equal to an arc of 114°. Sutures straight 
and horizontal. 
