CEPHALOPODA. 
239 
Orthoceras fluctum, n. sp. 
PLATE LXXVI, FIGS. 1-7. 
Shell straight, gradually enlarging from the apex to the outer chamber. 
Transverse section subcircular, or broadly elliptical. Apical angle 8°. 
Initial point unknown. 
Chamber of habitation large and regularly expanding towards the aper¬ 
ture, without any constriction so far as observed. Aperture unknown. Air- 
chambers numerous and comparatively shallow, having a depth of four or 
five mm. when the diameter of the tube is thirty to thirty-five mm., but 
often showing variation of depth in adjacent chambers. 
Septa thin, the concavity greater than the interspaces, being about seven 
mm., or equal to an arc of 100° to 108°. Sutures much curved, but not 
oblique to the axis. 
Siphuncle central or subcentral in well-preserved specimens, and small at 
its insertion in the septa, participating in the effects of pressure, as shown 
in fig. 7. Its passage through the interseptal spaces has not been traced; 
but in some specimens the convex sides of the septa present an elevated 
areola, with vascular markings surrounding the siplnmcular scar. 
There is rarely any evidence of test preserved, it having been dissolved 
and removed; and the surface-markings are unknown. The internal casts 
are essentially smooth, the interseptal spaces being flat, with an interme¬ 
diate groove made by the solution of the calcareous matter at the margins of 
the septa. The greatest length of the fragmentary specimens is about 200 
mm.; but the length of entire individuals has probably been twice as great. 
The diameter of the tube at the last septum, in a well-preserved specimen, is 
thirty-five mm. 
This species in many respects resembles the O. procerus; but it is less 
attenuate and proportionally less expanded in the outer chamber. The 
position of the siphuncle is also distinctive, while the septa, which are of the 
same frequency in both species, are in this one curved upon their exterior 
