stanton.] AMMONITES. 177 



At first the two outer nodes are equal, one being just on the angle 

 between the side and the abdomen and the other about halfway 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 larger example the 

 costae becomes distant, apparently by the suppression of the interme- 

 diate 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 convex, 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 serration equaling the costaB, though sometimes it is only slightly 

 sinuous and it is never completely divided into nodes as it is in Prion- 

 otropis woolgari. Septa very much like those of Prionotropis woolgari, 

 as is shown by the figures. 



None of the specimens show 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 that the 

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

 measures 47 mm in diameter; the outer whorl is 16 mm in height, exclusive 

 of the keel, and 15 mm in breadth. Some of the spines on specimens of 

 this size are 6 mm or 7 ,nm long. Some fragments apparently of this 

 species belonged to larger individuals. 



The development of this species is very much like that of P. ivoolgari, 

 but at the same time it shows many of the features of Prionocyclus 

 ivyomingensis. 



Ammonites meelcianus was described from fragments of the outer 

 volution of a specimen that seems to have had about the same form as 

 our species, but it was much larger than any of the examples I have 

 seen. The description of the septum shows that it had the same gen- 

 eral form as in P. hyatti, but the abdominal lobe is said to be broader 

 than the first lateral, while the opposite is true in ours. The species 

 has never been figured, and there are no specimens of it in the National 

 collection. 



I have not seen Amm. graysonensis, but judging from the description 

 and a figure of one of the types it might be founded on the young of 

 either this species or P. woolgari. The types of these two species came 

 from near Sherman, Texas, in beds probably equivalent with the Fort 

 Benton. 



Locality and position. — Abundant in the Pugnellus sandstone at many 

 localities in Huerfano park, Colorado, where the types were obtained. 

 A few specimens were found associated with the same fauna in the 

 u first and second ridges" of the section at Coalville, Utah. 



Bull. 106 12 



