WALKER ON ORIGIN AND DISTRIBUTION OF MOLLUSCA. 45 



tions duiing the various ages and the changes of physical geography 

 that they imply — the wonder is that so much has been accomplished u]) 

 to the present time. 



In no department of Zoology has better work been done than in the 

 molliisca and it is to a review of what has been accomplished towards 

 the elucidation of the origin and development of the existing non- 

 marine molluscan fauna of our own country, that I ask your attention 

 this evening. 



North America, north of Mexico, has usually been considered as a 

 distincc Zoological province, and forms the Nearctic Kegion of Wallace 

 and other earlier writers on the subject. It corresponds with the 

 Palaearctic Kegion of the old world, which embraces Europe, the Northern 

 part of Africa and Asia, north of the Himalayas. Later writers, on the 

 ground of "the absence of both positive and negative faunal characters 

 of sufficient importance to separate them from each other," have com- 

 bined these regions into one, extending around the entire northern 

 part of the globe under the name of the Holarctic Kealm. Be this as 

 it may, when viewed from the standpoint of the zoogeographer, whose 

 generalizations are based upon the fauna of all classes taken as a whole, 

 it must be admitted that from the standpoint of the conchologist, the 

 fauna of North America has many features, which stamp it with all the 

 indices of a peculiar region. Indeed under either scheme, the sub- 

 provinces are substantially the same. The main difference being, that 

 the Northern Province, so called, of America, is combined with the corres- 

 ponding region of the old world, into a single circumpolar region, as 

 the remaining subprovinces or regions remaining the same. With 

 this distinction in mind, it will be convenient, for the purpose of 

 'this paper, to follow substantially the arrangement of Binney who, 

 in studying the terrestrial mollusca, divided the continent into four 

 regions — The Northern. Interior, Californian, and Central. 



The Northern or Boreal Kegion comprises the entire northern por- 

 tion of the continent. Its southern border is not clearly defined. It 

 has been stated to be approximately fixed by the northern limits 

 of the cultivation of the cereals, and "may be indicated in general 

 terms as the same with the political division between the British posses- 

 sions and the United States, to the northeast corner of New York^ where 

 it runs southeasterly along the Appalachian chain of mountains to 

 Chesapeake bay." 



The Interior, or Appalachian Kegion, includes the entire eastern por- 

 tion of the continent, south of the Northern Kegion. and east of the 

 Sierra Nevada and Cascade mountains. From this, however, on the 

 south, may be separated such portions of Florida, Texas, Arizona, New 

 Mexico, Nevada and California, as from the admixture of tropical forms 

 seem better included in the Transition Kegion, so called, lying between 

 the Holarctic and Neotropical Realms. This includes the Southern 

 Region of Binney. The Central or Rocky Mountain Region, lies be- 

 tween the Sierra Nevada and Cascade mountains on the west, and the 

 Rocky mountains on the east. While the California or Pacific Region, 

 comprises the entire coast lands of the Pacific, west of the Rocky 

 mountains, and extending from Lower California to Alaska. 



The exact boundaries of these regions are often more or less indefinite, 

 except where natural barriers of ranges of mountains, deserts or great 



