CEPHALOPODA. 239 



Orthoceras flcctu.m, 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, beingabout 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 siphuncular 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 



