CEPHALOPODA. 
371 
Cyrtoceras aemulum, n. sp. 
PLATES XCVII, FIGS. 1-9; XCVIII, FIGS. 3, i. 
Shell slender, curved, making more than one-fourth of one volution. For a 
considerable distance back of the aperture the tube is but slightly arcuate, 
and becomes more curved toward the apex. Transverse section broadly oval 
or subcircular. Tube regularly and very gradually enlarging from the apex 
to the aperture, and without special contraction or expansion. Apical angle 
about 6°. 
The extent of the chamber of habitation has not been observed. The 
crenulations show, as obscure longitudinal ridges and striae, over the internal 
mould of the walls of the air-chambers. Aperture entire, with a sinus in 
the ventral margin having a width equal to about twice the depth. 
Air-chambers having a depth of seven mm., but showing considerable 
irregularity in different portions of the tube, and in different individuals. 
Septa smooth, with a concavity greater than the depth of the air-chambers. 
Sutures straight and horizontal. 
Siphuncle small, close to the ventral side, often exposed in the process of 
weathering. 
Test thin, having a thickness of about .5 mm. on the grand chamber. 
Surface marked by irregular, lamellose lines of growth. Tube ornamented 
with somewhat irregular, numerous, sharp, foliate expansions of the test, 
slightly inclining toward the aperture, and projecting about two mm. above 
the general surface of the test. The distance between two ridges varies 
in different specimens from two to three mm. Sinus sharp, deep, affecting 
the surface striae and ornaments, together with the annulations of the internal 
mould. 
Internal mould nearly smooth, but preserving traces of the crenulations 
and the external ornaments and markings of the test. Sutures moderately 
impressed. 
The largest fragment observed has a length of about 185 mm., with a 
diameter of thirty-two mm. at the aperture, and shows but a very gradual 
