CRETACEOUS FOSSILS. - 439 



Surface of the shell marked by fine, rather deep, longitudinal flutings 

 of varying width, with the ridges between very slightly flattened on the 

 surfaces; the whole crossed by extremly fine transverse strise of growth; 

 indicating the inner side of the apertural margin as the longest. Inner 

 surface of the tube smooth. 



Formation and locality. — In limestone concretions at the top of the gray 

 shales of the Fort Pierre' Group, on the Cheyenne Eiver, near Rapid Creek, 

 Black Hills. 



CEPHALOPODA. 



TETRABRANCHIATA. 



NAUTILID^E. 



Genus NAUTILUS Breynius. 

 NAUTILUS DEKAYI var. MOHTAN^NSIS. 



Plate 16, figs. 10-11. 

 Nautilus dekayi var. montancensis Meek, Pal. U. S. Geol. Surv. Terr., p. 498, PI. 27, fig. 2. 



Shell attaining a large size, subglobose in form, the length somewhat 

 greater than the width. Volutions rapidly increasing in size, rather sharply 

 rounded on the periphery, very slightly flattened on the sides, and again 

 more rapidly expanding near the umbilical region. Umbilicus closed, 

 though in exfoliated individuals appearing as if open by the removal of the 

 shell. Aperture deeply reniform, a little more than twice as wide as long 

 and sharply rounded at the sides Septa distant, deeply concave, their lines 

 of junction with the outer shell passing across the volution without curva- 

 ture. Siphuncle small; in some parts situated exactly central, and in others 

 nearly central between the dorsal and ventral borders of the septa. Outer 

 surface of the shell not observed,; but on the slightly exfoliated outer 

 .chamber seems to have been marked by concentric lines of growth directed 

 strongly forward on the sides, and on the dorsal region to have had a broad, 

 deep, retral curvature, indicating a deeply bilobed margin. 



The specimens of this variety are very closely related to N. dekayi 

 Morton, the differences not being sufficiently marked to be noted except on 



