GASTEROPODA. 
129 
ADDENDA TO THE GASTEROPODA. 
DESCRIPTIONS OF SOME NEW FORMS, AND NOTICES OF OTHERS PREVIOUSLY KNOWN. 
Since the commencement of this volume, additional information has been 
obtained regarding some of the species described, and a few new forms have 
been added to the Collection. The illustrations have been made as complete 
as the material available at the time would permit; but each new accession 
indicates the occurrence of forms not yet known or described. Much of this 
material is too imperfect for satisfactory determination, and farther collections 
are required for their study and proper arrangement. The great variation 
in form and other characters among the Platyceridae will always render their 
determination a matter of great difficulty, and many specimens must be left 
in uncertainty. A comparison of larger collections may result in showing 
that some of the forms which I have distinguished can be merged, for the 
lines of specific separation are not strongly marked, and these often become 
less striking and of less importance when compared among extensive collec¬ 
tions of individuals. 
The great variability of form and character among those designated as 
species has its maximum expression in the genus Platyceras, and continues 
as a marked feature in Platyostoma, which, in some species, is scarcely 
separable from the former genus by any persistent characters. This variable 
or erratic character is still manifested in the species of Stropiiostylus, where 
it is reduced to its minimum expression. 
In the higher groups the means of specific determination are better marked, 
but among these, so large a proportion of specimens is in the condition of 
casts of the interior that the determination often becomes very difficult and 
unsatisfactory. This is essentially true of nearly all the species in the 
Schoharie grit, and to a considerable extent among those of the limestones 
above. In this condition, many specimens are left undetermined and unillus¬ 
trated, such only among them having been selected as seemed important in 
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