Its 



Cyrtoceras Isodorxjs. (-N. sp.) 



Fig. 157. 



Fig. 158. 



Fig. 151 .—Cyrtoceras Isodorus. a, Section at the last septum ; b, side view. 

 158.— C. Huronense. a and b, Section at the last septum and side view. 



Description. — Shell rather strongly curved, laterally compressed. Sec- 

 tion ovate, the dorso-ventral diameter, at the last chamber, being to the 

 lateral in the proportion of ten to seven ; the greatest lateral diameter 

 nearer the dorsal than the ventral side ; the aperture slightly more 

 strongly compressed laterally than the septate portion ; the dorsal side 

 more obtusely rounded than the ventral. Septa one line distant on the 

 side. Siphuncle, small, in contact with the shell on the ventral side. 

 Chamber of habitation of moderate depth. 



The specimen is 2 inches in length measured along the curve of the 

 ventral side. Dorso-ventral diameter at the aperture 10 lines, lateral 

 6 (?) lines ; dorso-ventral diameter at last chamber 10 lines, lateral 7 

 lines ; dorso-ventral diameter at the eleventh septum 6 lines, lateral 

 5 lines. Depth of chamber of habitation 10 lines. The ventral contour 

 is curved to a radius of 15 lines ; the radius of the dorsal curve is about 

 12 lines. The siphuncle is a little less than a line in diameter, and appears 

 to be shghtly inflated between the septa. 



This species somewhat resembles O. simplex^ and indeed, fragments of 

 the two, if collected together, could hardly be separated. The former is 

 slightly more curved, more narrowly rounded on the ventral side, and 

 tapers more gradually than the latter. It is more cui'ved on the dorsal 

 side, and larger than C. Huronense. 



The aperture is not clearly shown in the specimen, but it is evidently 

 compressed laterally, and even seems to be (although obscurely) of the 

 key-hole shape of Phragmoceras prcematurum. 



Locality and Formation. — St. Joseph Island ; Black River or Trenton 

 limestone. 



Collectors. — A. Murray, R. Bell. 



