1888.] 467 [Newell. 



The shell is large, straight ; the transverse section is subcircu- 

 lar or oval from compression. The apical angle varies from 5° in 

 the nearly circular casts to 7£° in the more compressed forms. In- 

 itial extremity and living chamber unknown. 



The concavity of the septaequals an arc of from 97° to 110°, the 

 variation being largely clue to compression. The distance apart 

 of the septa is such that 2J to 3J of these spaces equal the diam- 

 eter of the shell. The sutures are nearly straight, but exhibit a 

 tendency to form broad lateral saddles. 



The siphon is large, central or slightly subcentral, varying from 

 subcircular to elliptical. The portions of the siphon between the 

 septa are not well preserved. Test unknown. 



These forms should be compared to 0. crebistriatum, 0. rectum, 

 O. unionensis, in Geology of Illinois, vi, p. 503, pi. 26. 



Locality. — Wabash Cit}^, Ind. 



Orthoceras unionensis Worthen. 

 Orthoceras unionensis Worthen, Geol. 111., vi, p. 505, pi. 26, fig. 4. 



Shell small, straight ; apical angle 3° in smaller part of shell ; 

 septa deep, subtended arc about 145° ; distance of septa equals 

 two-fifths of diameter of the shell. Test, living chamber, siphun- 

 cle and initial point unknown. 



Several casts are referred to this species, on account of their 

 small size and general outline. It is possible that this is a 3'oung 

 form of 0. crebescens. The best preserved cast measures in length 

 340 mm., diameterof largest closed chambers 29 mm., and diameter 

 of smallest 8 mm. 



Locality. — Wabash City, Indiana. 



Orthoceras rigidum Hall. 

 Orthoceras rigidum Hall, Pal. N. Y., in, p. 344, pi. 70, fig. 3. 



This species is represented by a fragment consisting of the cast 

 of eleven closed chambers, apical angle 7£° ; septa moderately con- 

 vex ; distant about one-seventh the diameter of the shell. Test, 

 siphon and living chamber unknown. 



locality. — Upper portion of the Niagara Group at Peru, Indiana. 



Orthoceras obstructum n. sp. 

 Shell straight, gradually enlarging ; transverse section broadly 

 elliptical ; apical angle 6° ; initial extremity and living chamber 

 unknown. 



