62 AN ANNOTATED CATALOGUE 



significance and importance, but on account of an entire 

 inappropriateness in a paper of this character of such pro- 

 lix discussions. That there exists a burdensome and ex- 

 tensive synonymy in some of the molhiscan groups, and 

 particularly in Unionidce, no student of fluviatile and la- 

 custrine mollusca will for a moment question ; in fact, so 

 manifest is it, to everyone who has given the subject even 

 a casual consideration, as to at once render apparent the 

 cogent necessity of a careful and complete revision of the 

 American species of this family. The wide geographical 

 distribution of some species of Uniones and the concomi- 

 tant changes of environment may readily be referred to as 

 among the chief causes of local variation of species. Not- 

 Avithstanding the careful and conscientious labors of some 

 of the earlier American writers on this group of mollusks, 

 the question of geographical distribution did not at that 

 time receive the attention that it has subsequently ; and 

 therefore species were often based upon superticial charac- 

 ters which are relatively unimportant as classificatory cri- 

 teria. 



Numerous references have been made in the following 

 notes to the occurrence of certain species in the loss ; and 

 there is appended a list of the moUuscan forms now known 

 from the post-pleiocene of Iowa. Most of the species 

 found fossil in this deposit are at the present day living in 

 the region under consideration ; the exceptions are notably 

 Patula strigosa Gould, Vcdlonia puldiella MiilL, Ilelicina 

 occulta Say, etc., — abundant and characteristic forms. 

 The first of these is now found no nearer than the Rocky 

 mountains ; the other two do not occur in central and other 

 portions of Iowa where the fossils are very abundant ; how- 

 ever, the former is present in a number of localities in the 

 eastern part of the state, but the latter is confined to two 

 very limited areas, and is on the verge of extinction. The 



