The Navarro group in east-central Texas is divided into four formations, in ascending order, Neylandville marl, Nacatoch sand, Corsicana marl, and Kemp clay. Recent paleontologic studies have resulted in the recognition in the group of 411 named species and varieties of the larger invertebrate fossils (exclusive of a few corals and crustaceans); in addition, more than 100 poorly and incompletely preserved forms are assigned to genera, but are too imperfect for specific identification. Of the 411 specifically named forms, 30 (7+ per cent) are known to range from beds of upper Austin age to the top of the Cretaceous. Fully 90 per cent of the Navarro fauna as at present known is restricted to the Navarro group and its equivalents. Only one species, Gryphaeostrea vomer (Morto ), is known to range upward into sediments of Tertiary age.

The Neylandville marl has yielded about 102 specifically named forms; leaving the 30 long-ranging species out of consideration, only 5 or 6 of the remaining species range downward into the Taylor marl, whereas 33 species range upward into the Nacatoch sand. From these figures it is concluded that the fauna of the Neylandville is more closely related to that of the Navarro group above than it is to the fauna of the Taylor marl below, and justifies classifying the Neylandville with the Navarro. The other formations of the group have yielded specifically named forms as follows: Nacatoch sand, 245; Corsicana marl, 98; and Kemp clay, 105.

The failure of practically all of the Navarro species, and 50 per cent or less of the genera, to appear in post-Navarro sediments is interpreted to indicate that a major geologic episode took place at the end of Cretaceous time. An uplift of the land or a lowering of sea-level, of sufficiently great magnitude to cause the strand line to retreat across the continental shelf to its relatively steep outer slope, thus restricting the area of sea bottom suitable for the shallower-water organisms to a very narrow zone, and the consequent severe competition for space and food, is suggested as an adequate cause to explain the extinction of many organisms and the rapid evolutionary development of others.

The Navarro group is correlated with the Maestrichtian of Europe mainly on the common occurrence of the cephalopod genera Sphenodiscus, Parapachydiscus, Discoscaphites, and Belemnitella. Representatives of these genera have been recorded from late Upper Cretaceous sediments in widely separated parts of the earth.