364 Jura^ Neoeomian and Chalk of Arkansas. — Marcou. 
Rase of Rocky Comfort chalk (Dakota-Niobrara epochs) — Ostrea 
bellaplieata Shumard. 
Ouachita Division. — Gryphsea sinuata var. americana Marcou; 
G. pitcheri, dilate var. Hill (not Marcou) ; Exogyra arietina 
Roemer ; Ostrea diluviana Lamk. 
Frederickburg Division. — Gryphica jntcheri (type) Morton; Ostrea 
flahellata Goldfus. 
Transitional Jura-Cretacic ( Trinity Division ) — Ostrea franklini 
Coquand. 
Jurassic? (Tucumcari beds) — Gryphica dilatataf Marcou; Ostrea 
marshii Marc. 
The horizontal line above indicates a great break. 
It is certainly a progress and a great one after the curious 
diagrams of Messrs. F. Roemer, James Hall and Benjamin F. 
Shumard ; but it shows how difficult it is to return to a good 
and logical classification, when an erroneous one has been 
used and maintained, by all means, against plain stratigraph- 
ical, palDeontologi(;al and lithological facts, during more than 
thirty years. 
Accepting the diagram of professor Hill, which is far more 
complete than I was able to make in 1853-61, for want of practi- 
cal detailed knowledge, because the reader must have always 
in mind, that when I made observations north of Texas, it was 
in a direct line and during a heavy march, under a military 
escort, and necessarily extremely limited ; only a sort of first 
glimpse at tlie geology of the 35th parallel — accepting that 
diagram, I shall give one, as I understand now the strati- 
graphic occurrence of the principal species of Ostrea. 
Tertiary, beginning with the Laramie group. 
Break.^ 
Fox Hill group or Albuquerque sandstone. ) 
Exogyra costata and Gryphxa vesicularis beds.> or Senonian. 
Exogyra ponderosa marls. ) 
Dakota group of Galisteo. — Ostrea congesta Conrad; or Turonian. 
Break. ^ . _ _ — — 
Greenish marly limestone of the junction of Little river with 
the Canadian river (Indian territory), or Cenomanian. 
Break . . 
Comanche series; Washita Division, or Albian, Aptian and 
Urgonian — Exogyra arietina Roem., Gryphxa sinuata var. 
americana. Marc, and Gryphxa n. sp. ind. called G. 
pitcheri, dilate var. Hill (not Marcou). 
Comanche series ; Frederickburg Division, or Neoeomian. — 
Gryphxa pitcheri Morton, Rcpmer and Marcou; Exogyra 
flahellata Goldfus. 
Break. 
Jurassic (Trinity beds) — Exogyra virgula Goldi., Ostrea frank- 
lini Coquand, Ostrea rcemeri Quenst. and GrypJiwa allied to 
arcuata. 
