254 
ORIGIN OF LIFE IN AMERICA 
I suggested above (p. 161) that the fresh-water mussels 
(Unionidae) probably effected their principal dispersal during 
the Mesozoic Era, and that this circumstance might account 
for the fact that we possess distinct proofs of a migration of 
species from North to South America. The great genus Unio 
has recently been subdivided by Dr. Simpson into numerous 
genera. One group of Unio (Plagiola), ranging from Mexico' 
to the Mississippi drainage Pasin, reappears southward in 
Nicaragua, another (Lampsilis) is known from Guatemala 
to Yucatan. Other groups of Unio are confined to South 
America. Unio-Tetraplodon occurs in Ecuador, having spread 
from there into the Amazon valley. Unio-Castalina lives in 
southern Brazil, Unio-Castaliella in Surinam and so forth. 
Finally Unio-Diplodon principally inhabits Chile, Argentina 
and Patagonia, while it reappears right across the Pacific 
in New Zealand and Australia. 
The range of these groups of Unio is apparently very com¬ 
plex in South America. Nevertheless, I quite concur with Dr. 
Simpson * in the belief that they all are the descendants of 
certain members of the family Unionidae, which wandered 
slowly from one river system into another, during the Triassic 
or some later Mesozoic Period, from North America to South 
America. To judge from the general distribution of the Unio¬ 
nidae in South America, they entered that continent from the 
west and only reached the eastern States subsequently. The 
group Unio-Hyria, as Dr. von Ihering f tells us, is nothing 
but a modified Unio, which has comparatively recently pene¬ 
trated from Guiana into Brazil. The most surprising fact 
which is so strongly brought out in that author’s remarkable 
researches is, that, while these Unionidae live in company with 
other families of fresh-water mussels in eastern South 
America, in Central America, Ecuador, Peru and Chile, that 
is to say westward of the Andes, Unios alone occur. This con¬ 
firms the opinion I expressed several times in previous chap¬ 
ters, that the faunistic interchange between North and South 
America took place between the western portions of the two 
continents. 
* Simpson, C. J., “Synopsis of the Najades,” p. 507. 
1 Ihering, II. von, “ Archhelenis und Arckinotis,” p. 122. 
