176 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA [bull. 106. 



regarded as a subgenus of Prionocyclus. When Neumayr described 

 the genus Acanthoceras, he cited A. woolgari 1 as an example of that 

 genus, but in the same paper he referred A. bravaisianns, which may- 

 be the young of the same species to ScMoenbachia, a genus that as 

 originally denned included Prionocyclus and Mortoniceras. In ZittePs 

 Handbuch der Paleontologie the same inconsistency is repeated, as A. 

 woolgari is listed under Acanthoceras, while both Prionotropis and Pri- 

 onocyclus are treated as synonyms of ScMoenbachia. On this point in 

 a personal letter to the writer, Prof. Alphens Hyatt says: " The type 

 of this genus [Acanthoceras] was in my opinion the Amm. angulicosta- 

 tus d' Orb. If this be granted, Prionotropis must be considered as the 

 type of another genus, since no one having the latest views with regard 

 to the Ainmonitinae would be apt to place it in the same group with 

 Acanth. angulicostatum unless unacquainted with the history of the 

 development of the individual in both of these types." Prof. Hyatt 

 regards Prionotropis as generically distinct from ScMoenbachia and 

 Prionocyclus also, although it seems to me to have much closer affini- 

 ties with the latter genus than with Acanthoceras. 



Locality and position. — Southeast base of the Black hills, South Da- 

 kota, and on the Missouri river 5 miles below the mouth of Vermilion 

 river in the Fort Benton shales. It is also reported from the same 

 horizon in northeastern Nebraska, in New Mexico, and 8 miles north 

 of Fort Lyon, Colorado, and from the Eagle Ford shales in Texas. 



In Europe the species seems to be confined to the Turonian. 



Prionotropis hyatti n sp. 

 PI. xxn, Figs. 5-8. 



Compare Ammonites meekianus Slmmard, 1860. Trans. St. Louis Acad, of Sci., vol. i, 

 p. 592, and Ammonites graysonensis Shnniard, ibid., p. 593. 



Shell of rather small size, compressed discoidal, consisting of five or 

 six whorls; volutions gradually increasing in size, embracing the ear- 

 lier ones but very slightly so that the umbilicus is broad, though dif- 

 ferent specimens vary somewhat in this respect. In very young exam- 

 ples the height of the whorls is greater than the breadth, the keel is 

 small, and more or less crenate, and the costae are simple, linear, and 

 strongly curved forward at the outer ends, without any nodes at first. 

 Usually every third or fourth costa is stronger than the others. Some 

 specimens three-fourths of an inch in diameter are scarcely distinguish- 

 able from the young of Prionotropis woolgari excepting that usually 

 the costae are slightly more unequal. As the shell continues to grow 

 the inequality of the costae becomes more marked and each of the larger 

 ones develops two nodes near the outer end where it curves forward, 

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



1 Deutscb, Geo!. Gesell. Zeitschr., vol. xxvn, 1875, p. 929. 



