LOWER CARBONIFEROUS FOSSILS. 575 



Formation and locality: Madison limestone, Crowfoot Ridge, Gallatin 

 Range, top of bed 24; J. P. Iddings. 



Form B. 



Shell very small, not spirally turned; of a conical or pyramidal shape, 

 much inclined. The front is edged or sharply rounded; the back, under 

 the apex, is flat, meeting the right side, which is more inflated than the left, 

 at an angle. The angle at which the posterior face meets the left side is 

 truncated. Peritreme subquadrate. 



Formation and locality: Madison limestone, north of Bighorn Pass, 

 Gallatin Range; A. C. Gill. 



Form C. 



Larger than either of the two already described, yet not very large; 

 much flattened transversely, very rapidly expanding. The right-hand face 

 of the fossil is somewhat flat; more inflated on the other side. Very 

 abruptly rounded in front and behind. But slightly inclined, so that a line 

 let fall from the apex onto the posterior extremity of the peritreme would 

 be nearly perpendicular to the plane of the base. 



Formation and locality: Madison limestone, north of Bighorn Pass, 

 Gallatin Range; A. C. Gill. 



Form D. 



PI. LXVI, figs. 12a, 126. 12c. 



Small, gradually expanding and closely inrolled for about one volution; 

 then very rapidly expanding for about half a volution, which is highly 

 inflated and not in contact with the involved portion. The latter is minute 

 and turned to the left. The peritreme is broken, but appears to have been 

 subquadrate. The sinistral side of the large whorl (that toward which the 

 involved apex turns) is somewhat flattened; the anterior end is sharply 

 rounded. Another angular turn, sinistro-posterior in direction, would bring 

 this subplanate face to the original one at an acute angle were not their 

 junction truncated or broadly rounded (!). Height about two-tliirds the 

 antero-posterior diameter. A line from the most posterior point of the 

 peritreme tangent to the involutions would be about perpendicular to the 

 plane of the base. 



