WALKER ON ORIGIN AND DISTRIBUTION OF MOLLUSCA. 57 



genera as they exist today are substantially the same as they were when 

 the race first appeared on the geological horizon, and this fact of great 

 antiquity is borne out by their worldwide and almost universal dis- 

 tribution at the present time. 



There is, however, a single genus of this class, whose peculiar distribu- 

 tion in recent times is one of the yet unsolved puzzles to the zoogeog- 

 rapher. This is a little group of limpet-shaped snails known as Gund- 

 lachia, originally discovered by the German naturalist, Gundlach, in 

 Cuba and named after him. Subsequent discoveries have shown that it 

 ranges north into the United States from Long Island to California and 

 south into South America. It is not found in any other part of the 

 world, except southern Australia, Tasmania and New Zealand. That 

 it spread into North America from the south is shown both by its 

 present distribution into that direction, and its absolute tailure to appear 

 either in former ages or at the present time in the fauna of Asia and 

 Europe. It is possibly one of the few survivors of that mighty army of 

 tropical forms which poured into North America in early Tertiary time 

 from the southern continent and which later perished so miserably upon 

 the advent of the glacial period. Its concurrent existence in South Am- 

 erica and Australia is very interesting and is one of the many evidences 

 both in fauna and flora which go to support the theory of the Antarctic 

 continent in Tertiary times. "A strip of land with a mild climate extend- 

 ing across the pole from Tasmania to Terra del Fuego would have 

 afforded a possible route * * * and the theory of a Mesozoic or older 

 Tertiary migration to or from Australia * * * would explain its present 

 position." Whatever may be the fate of the theory, the instance is an 

 interesting one, as exhibiting the methods by which modern science from 

 all possible sources — geological, palaeontological and biological — seeks to 

 reach the truth and reconstruct the history of the world. 



All of the existing families of fresh water, gill-bearing mollusca date 

 back to the era of the great inland lakes, which resulted from the separa- 

 tion of the Mesozoic sea from the adjacent oceans by the general conti- 

 nental elevation, which then took place. Not only every family, but 

 "almost every, if not every, genus and many of the subordinate divisions 

 of those genera, that are now among the living North American fresh 

 water mollusca, have been recognized among the species that constitute 

 the different faunae, the fossil remains of which have been collected from 

 the Mesozoic and Cenozoic strata of western North America." 



The present distribution of the two great families of Viviparidw and 

 Pleuroceridm which now constitute a most important feature of our 

 fauna, is in some respects quite dissimilar, and, while our present 

 knowledge of the distribution of these families and their progenitors in 

 time, is not sufficient to enable us to speak of with the same certainty 

 that may be done in regard to other groups, the facts, as they exist, give 

 rise to the same interesting speculations. 



The Yiviparidm are a family of almost universal distribution in the 

 northern hemisphere and of ancient lineage, dating back to Jurassic times 

 in both the old and new world. It is very abundantly distributed 

 through the eastern United States, but curiously enough, it is lacking 

 absolutely in the region west of the mountains along the Pacific coast. 

 Nor does it extend into South America. 



The Pleuroceridw on the other hand, while dating back at least, 



