Notes on Rirhinonl and Related Fossils 45 



hoyoiul which there is a slight reversal of curvature. This 

 causes the lateral parts of the se])ta to slojie moderately up- 

 ward from the dorsal toward the ventral side. 



The dorsal and ventral sides of the orthoceracone are 

 about equally convex. The radius of curvature equals about 

 seven-tenths of the chord connecting the ends of the arc 

 formed by the curvature. The lateral sides diverge at an angle 

 of about 7 degrees. The antero-posterior diameter of the shell 

 is '20 millimeters where its width is 53 millimeters. From this 

 point upward 10 chambers occur in a length of -IS millimeters. 



The siphuncle, or at least the cast of its interior, is num- 

 muloidal, the lateral diameter being 7 millimeters where the 

 shell is 53 millimeters wide. The more exact structure of the 

 siphuncle has not been determined. 



This species occurs in the Whitewater division of the 

 Richmond, at Richmond, Indiana, where the specimens here 

 figured were collected by John Misener. Similar specimens 

 occur in the Emswiler collection in the Museum of Earlham 

 College, at Richmond, Indiana, and are there numbered 8205 A, 

 8205 B, and 8206 A. 



In Bassler's Bibliographic Index of American Ordovician 

 and Silurian Fossils (Bull. 92, U. S. Nat. Mus., 1915), seven 

 species are listed under Tripteroceras. In six of these species, 

 including the genotype, Tripteroceras hastatum, Billings, the 

 ventral side is strongly flattened and the dorsal side is more 

 or less triangularly convex. However, in Tripteroceras Iambi, 

 \Miiteaves, these two sides are almost equally convex, the 

 transverse section is fusiform, the lateral angles are almost 

 acute, the siphuncle is nummuloidal, and the margins of the 

 septa are deeply concave on both the ventral and dorsal sides. 

 For this species the subgeneric term Lambeoceras is here pro- 

 posed. 



Tripteroceras richmondensis is closely related to Triptero- 

 ceras Iambi. The differences are slight. In Tripteroceras 

 richmondensis the angle of divergence of the lateral outlines 

 is less, and the septa are more strongly concave and form a 

 more acute angle with the lateral margins. 



