358 Jura^ Neocomian and Chalk of Arkansas. — Marcou. 
however, that "the lowest marine fauna of this division may- 
prove Jurassic affinities" {Amer. Jour. Sci., vol. xxxiv, p. 
306, October, 1887) ; but finally he created a division for it of 
the lower Cretaceous, which, according to his last view is 
composed of the Washita, Fredericksburg and Trinity divi- 
sions. 
Chapters xii and xiti are devoted entirely to the Trinity 
division, and are certainly the two most important chapters of 
the report. All is new there, stratigraphy and palaeontology. 
Professor Hill gives a detailed description of its stratigraphy 
in chapter xii, containing an important section, p. 119, of the 
gypsum bluff, two miles south of Murfreesboro, Pike county. 
The thickness of the division is 400 feet in average. It con- 
tains gypsum in several thin bands of a total thickness of only 
25 feet ; and the fossils are disseminated in blue and white 
marls, sometimes sandy, finishing at the base by an irregular 
deposit of lignite and bones of saurians. 
In Texas the Trinity division is much developed and covers 
vast surfaces. It lies in discordance of stratification upon the 
Palaeozoic rocks of Arkansas, and upon the Silurian, the Car- 
boniferous, the Dyas and the Trias of Texas. 
Chapter xiii is entitled : "Paleontology of the Trinity 
division." The molluscan fauna, which is the only one 
described, "bears remarkable resemblance to forms from the 
upper Purbeck and basal Neocomian, or Wealden beds of 
Europe." A review of all the species described and figured by 
professor Hill is necessary to arrive at the true age of this 
division. First we have Ammonites walcotti Hill, Plate i. 
The name is unfortunate because it creates a confusion with 
Ammonites loalcotii Sowerby, who has described under that 
name, in his celebrated Mineral conchyliology vol. ir, p. 7, 
Plate 106, 1815, an Ammonites of the upper Lias, with which 
the Ammonites of Arkansas has no affinities whatever. 
Professor Hill is right in saying that "it resembles Ammo- 
nites yo d'Orbigny, of the lower Neocomian ;" only the refer- 
ence to the lower Neocomian is an error, because d'Orbigny 
specially says that the Ammonites he describes under the 
name yo, belongs to the Kimmeridian of Boulogne-sur-Mer. 
The species of Mr. Hill is new ; but is closely allied to 
Ammonites yo., being much smaller, and the ombilix is not so 
narrow. It resembles Ammonites litocerus Oppel, with its 
