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 

 us 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 Strophostylus, 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 

 ! s 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 

 17 



