172 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA. Lbull.ioc 



The breadth of the whorls is about two-thirds of the height in young 

 specimens; in larger ones it is proportionally somewhat greater. 



The species sometimes reached a size not less than 25 cm in diameter. 

 The outer whorl of a specimen that is 125 mm in diameter measures 38 mm 

 in width by 48 in height. 



Ammonites germari Reuss, which occurs in the Turonian of Germany, 

 seems to be a closely-related species. 



Locality mid position. — The types came from the valley of the Medi- 

 cine Bow river, Wyoming; Whitfield's specimens were found "in sili- 

 ceo-caleareous layers of Division No. 2, Cretaceous, on the east fork of 

 Beaver creek, near Camp Jenney, Black hills." It is common but usu- 

 ally not well preserved in a layer of calcareous sandstone immediately 

 beneath the Niobrara limestone in Huerfano park and other places in 

 southern Colorado and along the eastern base of the Front range, and 

 it occurs in eastern Utah. 



Prionocycltjs macombi Meek. 

 PL xli, Figs. 1-5. 



Prionocyclus? maconibi Meek, 1876, Macomb's Exploring Exped. from Santa F6, N. 

 M., to the Junction of Grand & Green Rivers, Geol. Rept., p. 132, PI. 2, Figs. 

 3a-d. 



Original description: 



"Shell discoidal; umbilicus shallow; somewhat less than the diam- 

 eter of the last whorl from the ventral to the peripheral side, and show- 

 ing all the inner turns; volutions increasing gradually in size, very 

 slightly embracing, compressed so as to be nearly flat on the sides, but 

 rounding into the umbilicus; periphery rather narrow, nearly flat, and 

 provided with a small mesial carina, which is very slightly waved in 

 outline; lateral margins of the periphery each having a row of small 

 compressed nodes, arranged one at the termination of each of the eostae, 

 with their long diameters nearly parallel to the peripheral keel; sides 

 of each turn ornamented by from thirty- six to forty rather obscure, 

 slightly flexuous costae, only every second, third, or fourth one of which 

 extends across to the umbilical margin, where they are usually a little 

 swollen. 



" The septa are generally a little crowded in adult shells, and divided 

 into two very unequal principal lobes on each side. Siphon al lobe 

 slightly longer than wide, and ornamented by three branches on each 

 side, the two terminal of which are a little larger and much less spread- 

 ing than the lateral pair, and each ornamented by some five or six 

 sharp digitations along the margins and at the extremity, while the 

 first pair of principal lateral branches above the terminal ones are of 

 nearly the same form as the latter, but more spreading, and the third 

 pair are smaller and merely provided with a few digitations; first lat- 

 eral sinus (dorsal saddle of old nomenclature) as long as the siphonal 



