Jxcra, Neoeoniiav and Chalk of Arkansas. — Marcou. 363 
Nucula, allied to N. rostralis of the upper Lias of Salius (Jura). 
Cyrena (Corbiculaf) arkanmensu Hill. A Coring closely allied to 
C. crenala Cont. Kimmeridian. 
Corbiculaf (Aslarte?) pikeiisis Hill. It is an A.startc. 
Cardium? sevicriensis Hill. 
Anomia. sp. ind. Hill. A yenus very eommon in the upper Jura. 
Professor Hill is inclined to regard this invertebrate fauna 
as a brackish water fauna, going so far as to say that "the 
shells are all of forms whose living representatives are known 
to inhabit brackish waters," and comparing them to forms 
from the upper Purbeck and basal Neocomian or Wealden 
beds of a part of central Europe. I can not agree either with 
the characters of brackish fauna or the Wealden or Neocomian 
age of the Trinity formation. The molluscan fauna is a 
marine fauna, littoral to be sure, but no mixture of fresh 
water forms exists so far. As regards the age, we have in 
Texas and Arkansas the true equivalent and homotaxis of the 
Neocomian and Wealden of central Europe in the Frederick 
division of the Comanche series of professor Hill ; and the 
Trinity division is older, corresponding with the upper Juras- 
sic, from the Purbeck included down to the Oxfordian. 
Chapter xv, "Resume of the Cretaceous group" of profes- 
sor Hill's important volume, deals mainly with what he calls : 
"two misconceptions concerning the Cretaceous formation," 
giving excuses for the opinions expressed repeatedly and 
maintained contrary to the facts observed and recorded by me 
as far back as 1853, and condensed a last time in the clearest 
way in my paper of 1861 : "Notes on the Cretaceous and 
Carboniferous rocks of Texas" {Proceed. Boston Soc. Nat. 
Hist. vol. viir, p. 86), where will be found an exact section of 
the Cretaceous strata of Texas, showing that in America we 
have a complete series of strata of the lower Cretaceous or 
Neocomian and Aptian, middle Cretaceous or Green Sand and 
Turonian, and upper Cretaceous or Senonian ; only I did not 
give the details, which were unknown to me. 
On p. 166 professor Hill gives a diagram of the stratigraphic 
occurrence and range of the principal species of the genus 
Ostrea. It is a very instructive table : 
Exogyra coslata green sand. — Gryphcca vesicidarh Lamk. 
Ezogyra ponderosa marls. 
Rocky Comfort chalk. No Ostrea. 
