256 PROCEEDINGS OF THE ACADEMY OP 



more rapid expansion from the smaller to the larger extremity, and in its com- 

 pressed instead of cylindrical form, as well as in having its striae passing di- 

 rectly around, instead of obliquely. 



Locality and jposition. Joliet, Illinois ; Niagara Group, of Upper Silurian 

 series. 



ORTHOCERAS SlTBBACtJLDM, M. & W. 



Shell attaining a moderately large size, slender and gradually tapering ; 

 entire length, as inferred from the convergence of the sides towards the small- 

 er extremity, about thirty inches ; section elliptical, the smaller diameter be- 

 ing to the larger as about 17 to 24. Length of specimen, imperfect at both 

 extremities, 18 inches, of which the outer or body chamber occupies a length 

 of 7"50 inches, while twelve of the smaller chambers occupy the remaining 

 10 - 50 inches ; greater diameter at larger end, 3*23 inches ; smaller do. of same 

 about 2*10 inches ; greater diameter at smaller end, 1*90 inches ; smaller do. 

 of same, 1*45 inches. Septa transverse, rather deeply concave, separated by 

 chambers, two and a half of which equal the greater diameter of the shell at 

 the point of measurement. Surface and siphon unknown. 



At a first glance this species looks much like 0. Bigsbyi, Stokes, [= Ormo- 

 ceras tenuifilum, Hall], from the Trenton and Black River Limestones, but on 

 a closer inspection it is found to be more compressed and more gradually ta- 

 pering, while its septa are considerably more distant, and show no backward 

 curve on either side. 



Locality and position. Joliet, Illinois. Niagara division of Upper Silurian 

 series. 



Orthoceras Jolietense, M. & W. 



Shell much elongated, very gradually tapering ; section oval or narrow el- 

 liptic, (probably to some extent, at least, due to accidental pressure) ; septa 

 very concave, unusually distant or separated by spaces, equalling three-fourths 

 the greater diameter of the shell at the point of measurement. Siphon and 

 surface unknown. 



Length of a septate specimen, imperfect at both extremities, 14*50 inches ; 

 greater diameter of do. at larger end, 2 - 75 inches ; smaller do. of same, 1*77 

 inches; greater diameter at smaller end, 2-16 inches; smaller do. of same, 

 1*30 inches. Number of septa in the entire 14J inches, 8. 



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

 ally distant septa. The latter character will alone distinguish it from any 

 Upper Silurian species known to us, excepting 0. pauciseptum, Hall, from the 

 Shaly Limestone, of the Lower Heldeburg Group. From this New York species, 

 to which it seems to be nearly related, it will be distinguished by its com- 

 pressed, 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 Silu- 

 rian series. 



Orthoceras nobile, M. & W. 



Shell attaining a very large size, rather rapildy tapering ; section subcircu- 

 lar, or very slightly flattened on one side. Septa deeply concave, extremely 

 thin, distant about one-fifth the diameter of the shell at the point of measure- 

 ment ; siphon central, round. Surface unknown. 



Entire length of a septate specimen incomplete at both ends, 18 inches ; great- 

 est diameter at larger end, 8| inches ; smaller do. of same, 7 inches. Greatest 

 diameter at smaller end, 5 inches : angle of divergence about 14 ; diameter 

 of siphon at larger end, 0*80 inch. The entire length of the shell was pro- 

 bably not less than five feet, and its body chamber, owing to the large size and 

 rapid expansion of the shell, must have been very capacious. 



[Dec. 



