This paper touches upon the growing importance of paleoecology. The classification of marine environments now in vogue with biologists is briefly reviewed and modified for geological and paleontological purposes. Two classifications are recognized. The habitat classification (planktonic, nectonic, necto-benthonic, benthonic) is used without reference to depth of water involved. It is concluded that most or all shelled cephalopods were mobile benthonic and necto-benthonic. The bathymetric classification refers to depth of bottom. Divisions recognized are: 1. littoral, 2. neritic (a. mud flat, b. reef, c. epineritic, d. infraneritic), 3. bathyal (a. epibathyal, b. infrabathyal), 4. abyssal.

It is suggested that the more obvious factors controlling ammonoid distribution may be interpreted in terms of morphology of shell, habits, and distribution of living cephalopods, associated fossil faunas, types of sediments, paleogeography, bottom, and factors of the water medium such as salinity, temperature, depth, oxygen content, waves, currents. Flotation of shells after death of the animal probably occurred, but was much less important as a factor of distribution than is commonly supposed.

On the basis of shell morphology the principal Cretaceous ammonoid groups recognized are: 1. smooth, involute, obese (Phylloceras), 2. smooth, evolute, obese (Lytoceras), 3. smooth, ovate (Desmoceras), 4. sculptured, ovate to quadrate, a medium sculpture (Dufrenoya), b. robust sculpture (Mortoniceras), c. obese, spinose (Neocardioceras), 5. tenuous, involute, lightly sculptured (Oxytropidoceras), 6. tenuous, involute, smooth (Engonoceras), 7. uncoiled types (several subdivisions, Turrilites, Hamites, et cetera.)

It is noted that ammonoids do not occur in littoral, mud flat, or reef deposits, and are rare with Orbitolina, rudistids, and corals. Only the tenuous ammonoids (Engonoceras, Oxytropidoceras) are numerous with the abundant echinoids, oysters, and thick-shelled gastropods and pelecypods of the epineritic zone. Sculptured forms of great variety and abundance occur in sediments deposited in the infraneritic zone (estimated at depths of 20-100 fathoms). Smooth, ovate genera (Desmoceras) lived in the epibathyal zone at or just below 100 fathoms. Smooth, obese genera (Phylloceras, Lytoceras and related forms) are found to the exclusion of other fossils in sediments of the infrabathyl zone. The Texas Cretaceous seas were too shallow to attract the smooth obese genera.