Abstract

A large collection of Turonian ammonites from the Ojinaga Formation of northern Chihuahua and from the Eagle Ford Group of central Texas contains species of the family Collignoniceratidae that permit improved definitions of the genera Prionocyclus and Selwynoceras, but transitional forms will not be eliminated by any diagnosis. Collignoniceras Breistroffer is a synonym of Selwynoceras Warren & Stelck. The "Spathite zone" in Chihuahua has yielded a new species, Mammites? depressus of the family Acanthoceratidae, a new species, Spathites rioensis, of the family Vascoceratidae, as as well as species of the genera Neoptychites, Coilopoceras and Romaniceras.

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