Upper Cretaceous rocks in West Texas and northeastern Chihuahua, Mexico, are divided into the Ojinaga, San Carlos, and El Picaco Formations.

A quantitative zonation based on morphologic changes in stratigraphically successive Placenticeras samples is proposed to supplement the established Upper Cretaceous collignonicerid zonation. The correlation coefficients of eight stratigraphically unrelated samples are compared statistically with the established Placenticeras evolutionary sequence and time correlations are made.

The results of the biostratigraphic analysis suggest that the Senonian strandline in Presidio and Jeff Davis Counties, Texas, and northeastern Chihuahua shifted southeastward during an extensive marine regression.

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