426 VALJBOSTOLOQY OF NEW YORK. 



S pta regular and slightly advancing from the centre of the dorsum to the 

 umbilical angle; thence curving backward, and again forward, they are 

 recurved from the ventro-lateral angle, making a gentle inflexion across the 

 lateral face of the volution, and a more abrupt, retral curve on the ventral 

 face. This direction of the septa corresponds very nearly with their direc- 

 tion in the simpler forms of Goniatites, presenting a low saddle on the 

 ventro-lateral and dorso-lateral angles, and a broad, sballow lateral, and nar- 

 rower ventral lobe. Convexity of the septa, measured across the dorso- 

 ventral diameter, 112°. Siphuncle small, near the convex ventral side; its 

 elements not fully known. 



Test and surface-markings unknown. 



The specimen described is an internal cast, measuring across the disc 

 230 mm.; the dorso-ventral diameter of the outer volution at the base of the 

 air-chamber is sixty-five mm. 



This species bears considerable general resemblance to N. (Discites) Marcel- 

 lensis, but is distinguished by its larger size, re-entrant volutions, form of the 

 transverse section, more gradually enlarging tube, and less expansion of the 

 grand chamber. 



Formation and locality. The specimen was found in a loose mass of limestone 

 of the Upper Helderberg group (the original source of which is unknown), at 

 Ann Arbor, Michigan. 



Nautilus (Discites) inopinatus, n. sp. 



PLATE CX, FIGS. 1, 2. 



Shell discoid, the greatest thickness of the disc being a little less than half the 

 greatest transverse diameter. Volutions three or four, contiguous ; some- 

 what gradually expanding in the inner volutions, and much more rapidly in 

 the outer one. Umbilicus wide, showing the entire lateral faces of the 

 volutions. Transverse section quadrangular, trapezoidal, the base shorter 

 than the sides; sides nearly straight, and gradually diverging from the base; 

 peripheral margin curved, convex outwardly, and longer than the side. Tube 



