CEPHALOPODA. 297 



mm., marked by a cruciform cicatrice, which is formed by two unequal pairs 

 of oval furrows, extending to a central node and forming the umbilicus. 



Chamber of habitation small, cylindrical ; length about two and a half 

 times the diameter at the last septum. Aperture entire, without special con- 

 traction or expansion. Air-chambers regular, numerous, more frequent than 

 the annulations, with a depth of about three mm. 



Septa smooth, thin. Their concavity is unknown, on account of the great 

 degree of compression to which most of the specimens have been subjected. 

 Sutures straight and horizontal, and bearing no uniform relation to the fre- 

 quency or curvature of the annulations. 



Siphuncle subcentral, moniliform, slightly indenting the septa. Its diameter 

 at the septa is about two mm., where the tube has a diameter of seven- 

 teen mm. 



Test very thin, rarely preserved. Tube ornamented with prominent annu- 

 lations, which become more frequent and less marked toward the apex. On 

 the chamber of habitation the annulations are very sharp and elevated, often 

 curved and oblique. The tube, for a distance of twenty mm. or more back 

 of the aperture, is cylindrical and without annulations, but is marked by 

 lamellose lines of growth. Surface marked by fine, regular, sharp, con- 

 tinuous longitudinal .striae, crossed by finer, crowded, irregular striae. There 

 are about fifteen to twenty of the longitudinal s trite in the space of two mm. 



The internal mould is essentially smooth, with the exception of the promi- 

 nent annulations, and sometimes shows lamellose-striate markings. 



A fragment, having a diameter at the larger extremity of sixteen mm., 

 and a length of eighty-five mm., has sixteen annulations. The apical por- 

 tion of another individual, with a diameter of ten mm. at the larger 

 extremity, shows twenty-five annulations in the length of fifty-five mm. 

 A fragment of a larger individual has only six annulations in the same 

 space. 



This species is distinguished from 0. Thoas, of the Schoharie grit and Upper 

 Helderberg limestones, by the character of the surface-markings and the rapidly 

 enlarging tube ; and from 0. nuntium by its more prominent and less frequent 

 38 



