Logan.] The Invertebrates , Benton Group. 469 



an inch in diameter are scarcely distinguishable from the young 

 of Prionotropis woolgari, excepting that usually the costse are 

 slightly more unequal. As the shell continues to grow the 

 costs? become more marked and each of the larger ones de- 

 velops two nodes near the outer end, where it curves forward, 

 and on some of them there is also an elongated node near the 

 umbilicus. At first the two outer nodes are equal, one being 

 just on the angle between the side and the abdomen and the 

 other about half way between it and the keel, but at a later 

 stage the nodes nearest the keel become obsolete, while the 

 others rapidly increase in size, and some of them are developed 

 into prominent sharp spines that are directed obliquely out- 

 ward and backward. On the outer whorl of the large example 

 the costie become distant, apparently by the suppression of the 

 intermediate smaller ones. While these changes in the surface 

 ornamentation are developing, the form of the volution is also 

 considerably altered. The abdomen becomes flattened on each 

 side of the narrow, prominent keel, the sides become less con- 

 vex, and the breadth of the whorl is finally almost equal to the 

 height, so that its cross-section is subquadrate. The keel in all 

 the larger specimens is usually more or less serrate, the serra- 

 tion equaling the costre, though sometimes it is only slightly 

 sinuous, and it is never completely divided into nodes as it is 

 in Prionotropis woolgari. Septa very much like those of Priono- 

 tropis woolgari, as is shown by the figures. None of the speci- 

 mens shows the complete living chamber, but the well-developed 

 spines and other features of the surface ornamentation seem to 

 be adult characteristics, and it is therefore probable the species 

 never attained a very large size. The largest one before me 

 measures forty-seven mm. in diameter; the outer whorl is six- 

 teen mm. in height, exclusive of the keel, and fifteen mm. in 

 breadth. Some of the spines on specimens of this kind are 

 six mm. or seven mm. long. Some fragments apparently of 

 this species belong to larger individuals." 



Numerous impressions of forms which are undoubtedly of 

 this species occur in the Fort Benton limestone in the Salt 

 creek and Rattlesnake creek outcrops. 



