FRESH-WATER MUSSELS 
161 
entirely different.* * * § 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,*}* 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 Mesozoic 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 $ 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. Sirnp- 
* White, Charles A., “ Ancestral Origin of North American Unionidae,” 
pp. 77—79. 
f Ihering, H. von, “ Najaden von S. Paulo,” pp. 133—140. 
J Whitfield, K. P., “Fossil Unionidae from Laramie Clays,” p. 624. 
§ Ihering, H. von, “ Anodonta and Glabaris.” 
L.A. 
M 
