44 C. A. White—Fresh-water and Land Mollusca. 
orado or New Mexico, every condition which experience points 
out as favorable. We shall find in the same territory con- 
is dependent upon a judicious choice, the preceding observa- 
tions, I hope, may in spite of their incomplete ness, furnish use- 
ful material for a more exact qualitative comparison, than has 
heretofore been practicable. 
Art. IV.—On the Antiquity of Certain eg ele Types of 
Fresh-water and Land Mollusca ; by C. A. Watts, Paleon- 
tologist to the U. S. National Museum 
AMONG existing fresh-water and land Mollusca there are cer- 
tain comprehensive genera, which may be divided into a greater 
or less number of more or less distinctly definable groups, that 
are respectively recognizable by certain common characteris- 
tics, less conspicuous than those which separate the larger gen- 
era from each other. These minor groups have been treated 
as genera, sub-genera, or as still less important sections, by the 
various authors who have discussed them, according to the 
individual estimate that has been placed upon the relative 
value of the characters by which they are ene. It is 
my present purpose, not to discuss the value of these distine- 
tions as means of zoological classification, but to show that a 
considerable number, not only of the larger genera of living 
North American fresh-water and land Mollusca, but also a 
early as the Sepocka 6 epochs of the Cretaceous, or the imme- 
remarks, are those which have esti obtained by the different 
U. Government Surveys in the western portion of our na- 
tional domain. The strata which have furnished these fossils 
are, in the ascending order, those of the Fox Hills, Laramie, 
Wahsatch, Green River and Bridger groups. The first named 
of these groups is unquestionably Cretaceous, and the three 
last are as unquestionably Eocene Tertiary. The second I 
regard as representing a transitional epoch, but some penlogtita 
assign it to the Cretaceous period because of the presence of 
agg Iaees remains in its strata. Others refer it to the Ter- 
iary, because of the characteristics of its floral remains. It is 
scent for my present pu e to say that the molluscan 
here discussed are Sat gy strata which range from the 
emsects to the close of the Hocene, inclusive. 
