YEETEBEATES. 465 



Orthoceeas Unionensis, Worthen. 



PI. 26, Fig. 4. 



Shell below the medium size, rather rapidly tapering, especially 

 along the upper half of its septate portion, and more gradually below, 

 slightly arcuate, (though this may be due to accident,) septa transverse, 

 rather deeply concave, distant on the upper half of the shell a little less 

 than one-half the greatest diameter at the point of measurement, sur- 

 face markings and siphuncle unknown. 



This species differs from the last in its more slender and more rapidly 

 tapering form, and the comparative distance of its septa. Length of 

 the septate portion of the shell in the specimen under examination, 

 9 inches ; diameter at the base of the chamber of habitation 1.50 

 inch; diameter at the lower extremity, 0.45 inch. 



Locality and position : Union county, Illinois, from the red layers at 

 the base of the Niagara, Upper Silurian. 



Oethoceras Jolietensis, M. and W. 



PI. 20, Pis. 5. 

 Orthoeeras Jolietensis, Meek and Worthen, 18C5, Procoed. Acad. Nat. Sci. Pliilad., p. 256. 



Shell mucli elongated, very gradually tapering, section oval or narrow 

 elliptic; septa very concave, unusually distant or separated by spaces 

 three-fourths the greater diameter of the shell at the point of measure- 

 ment. Siphuncle and surface unknown. 



Length of a septate specimen, imperfect at both extremities, 14.50 

 inches ; greater diameter of septate specimen at larger end, 2.75 inches ; 

 smaller diameter of same, 1.77 inch ; greater diameter at smaller end, 

 2.16 inches ; smaller diameter of same, 1,30 inch. "Number of septa in 

 the entire 14i inches, 8. 



This species is remarkable for its very gradually tapering form, and 

 unusually distant septa. The latter character will alone distinguish it 

 from any Upper Silurian species known to us, excepting O. paueiseptum, 

 Hall, from the shaJy limestone of the Lower Helderberg Group. Erom 

 this New York species it will be distinguished by its compressed instead 

 of cylindrical form. It is true this compression may be in some degree 

 due to accidental pressure, but it seems to be too regular along the entire 

 length of the shell not to be mainly the natural form. 



Locality and position : Joliet, Illinois. Niagara division of the Upper 

 Silurian series. 



