FEESH-WATBR MUSSELS 161 



entirely diSereot.* The geographical distribution of fresh- 

 water mussels in North America thus constitutes a practical 

 demonstration of the correctness of the view so ably main- 

 tained by Dr. von Ihering,f and supported by Dr. White, 

 that these mollusks are not appreciably affected by chance or 

 accidental dispersal. 



The family Unionidae, to which all the North American 

 fresh-water mussels belong, first appeared during the Triassic 

 Age. Their principal diffusion over the globe may possibly 

 have been effected in Secondary or Mesozoio times. During 

 the closing period of the Mesozoic Era, the Cretaceous, the 

 family attained an extraordinary development, particularly 

 in the Laramie strata. The remarkable feature is that many 

 of the species in these beds are so nearly like the living 

 species that according to Professor Whitfield if they are to all 

 intents and purposes the same. There were at that time 

 (the Cretaceous Period) two great land masses in place of the 

 North American continent viz., one in the east, the other 

 in the west. The fresh-water mussel fauna occupied then 

 as far as we know, mainly the eastern flank of the western 

 land-mass. The latter was probably connected, as I shall 

 endeavour to show later on, with some old land-masses on 

 the west coast of South America. South America may thus 

 have acquired its Unionidae in Mesozoic times. 



The two great families of fresh-water mussels, Unionidae 

 and Mutelidae, have been recognised for some time past, also 

 the restriction in distribution of the latter family to Africa 

 and South America. It was not until the year 1891 that 

 Dr. von Ihering § made the striking discovery that all the 

 Unionidae begin their existence on hatching from the egg, as 

 so-called " glochidium " larvae, while the Mutelidae have an 

 entirely different "lasidium" larva. The Unionidae, of 

 which about one thousand species are now known, have since 

 been subjected to a thorough critical revision by Dr. Simp- 



* WMte, Charles A., "Ancestral Origin of North American Unionidae," 

 pp. 77—79. 

 t Ihering, H. von, " Najaden von S. Paulo," pp. 133—140. 

 X Whitfield, E. P., " Fossil Unionidae from Laramie Clays," p. 624. 

 § Ihering, H. von, " Anodonta and Glabaris." 

 L.A. M 



