STANTON.] 



MORTONICERAS. 179 



closely. Both it and Ammonites woolgari have been referred to Acan- 

 thoceras by some authors, while other species that seem to me related 

 to it, such as Amm. michelinianus and Amm. regular is have been assigned 

 to Hoplites. 



P. Icevianus is included in the fauna of the Colorado formation be- 

 cause no Ammonites of the same type are known in higher strata and 

 because fragments that are believed to belong to this species have 

 been found in the Fort Benton shales of Colorado. 



Genus MORTONICERAS Meek. 



MORTONICERAS SHOSHONENSE Meek. 



PI. xliu, Figs.- 1 and 2. 



Mortoniceras shoskonense Meek, 1876, U. S. Geol. Sur. Terr., vol. ix ; p. 449, PI. 6, Figs. 

 3a, c and 6b. 



u Shell coinpressed-discoiclal, with umbilicus apparently nearly or 

 quite twice as wide as the outer whorl ; volutions very narrow, with 

 dorso-ventral and transverse diameters equal, and section subquad- 

 rangular, those within scarcely one- sixth embraced by the succeeding 

 turn; costa3 each mainly represented by two nodes, the inner of which 

 is low, compressed, and elongated so as to extend from near the um- 

 bilical margin about halfway across the sides, while the outer near 

 the peripheral margin are more prominent, rounded, and directed lat- 

 erally; keel less prominent than the row of compressed nodes on each 

 side about halfway between it and the rounded nodes along the mar- 

 gins of the periphery; compressed nodes on the periphery of each inner 

 turn covered by the succeeding volution, the inner margin of which is 

 indented by the rounded lateral nodes of that next within. 



u Septa moderately approximate; siphonal lobe oblong, about once 

 and a half as long as wide, with small, short, nearly parallel, serrated 

 terminal branches, and three or four very short, digitate, and simple 

 bran chiefs and points on each side ; first lateral sinus wider than the 

 siphonal lobe (which it equals in length), unequally bipartite at the 

 anterior end, both divisions being digitate, and the larger one on the 

 siphonal side deeply bifid; first lateral lobe somewhat longer, but nar- 

 rower than the siphonal, and having its terminal division deeply bifid, 

 and its lateral margins bearing a few very nearly simple branchlets; 

 second lateral sinus scarcely more than half as wide as the first, and 

 much shorter on the umbilical side, unequally bifid or trifidat the end, 

 with more or less sinuous margins; second lateral lobe only about half 

 as long and wide as the first, and trilobate, with the small middle divi- 

 sion emarginate at the end; third lateral sinus a little shorter and nar- 

 rower, and irregularly tridentate at the end; antisiphonal lobe about 

 as long as the first lateral, but narrower, with a few short, nearly sim- 

 ple, lateral divisions, and a tridentate posterior extremity. 



