31 



the shell, which forms, as shown in a transverse section, the nar- 

 row end of a sharply ovate figure, at the ventral margin. The 

 sharply ovate transverse section is represented in figure VI. 

 Twelve septa have a length, on the inner curve or dorsal side, of 

 six-tenths of an inch, and, on the ventral side of one and eight- 

 tenths inches, where the dorso- ventral diameter, at the smaller 

 end, is half an inch, and, at the larger end, nine-tenths of an 

 inch. The greatest lateral diameter is about the dorsal third of 

 the shell where it measures, at the same sections, thirty-five hun- 

 dredths of an inch, at the smaller end, and eighty- five hundredths 

 of an inch, at the larger end. The septa cannot be distinguished 

 near the apical end of our specimen. The chamber of habitation 

 is, probably, nearly complete, in our specimens, and it constitutes 

 more than one-third of the entire length of the shell. Part of the 

 shell is preserved on the inner dorsal side and shows regular 

 lamellose lines of growth, without other ornamentation. Part of 

 the shell is also preserved over the siphuncle and posterior ven- 

 tral side of the body chamber, which shows the lamellose lines of 

 growth curve backward, in crossing the siphuncle, and indicates a 

 notch or sinus at the ventral lip of the aperture. Where the 

 shell is decorticated, the cast is smooth. 



This species is distinguished by its strong curvature, rapid en- 

 largement, sharply ovate transverse section, long body chamber 

 and lamellose lines of growth. 



Found in the Trenton Group, at Dunleith, Illinois, and now in 

 the collection of Wm. F. E. Gurley. 



Family OETHOCERATID^. 



ORTHOCERAS CALDWELLENSIS, n. Sp. 



Plate IV, Fig. 1, middle part of a specimen; Fig. 2, transverse 



section. 



Shell straight, large, long, very slowly and regularly enlarging 

 from the apex toward the mouth of the chamber of habitation. 

 Only the middle part of the shell is preserved in our specimens. 

 Chamber of habitation unknown. Transverse section subelliptical. 

 Siphuncle subcentral. The shell is preserved, on our specimen, 

 and the air chambers are not, therefore, exposed. The shell is 

 widely and deeply annulated or transversely furrowed. The di- 

 viding ridges are nodose. The nodes are arranged in longitudinal 



