Occasional Papers of the Museum of Zoology 5 
of water, near the town of Catemaco, and about a hundred 
yards from the lake, which fills the crater of a small subsidiary 
cone. 
ACKNOWLEDGMENTS 
I wish to add my thanks to those of Dr. Ruthven, already 
' expressed, to all of the Americans at the Haciendas of Cuato- 
tolapam and Hueyapam, for their kind hospitality and gener- 
ous assistance. In addition, I wish to express the greatest 
indebtedness to Dr. Ruthven and wife, for help in every way. 
As already mentioned, I owe the very production of the 
paper to the generous and patient advice and instruction of 
Dr. H. A. Pilsbry. Mr. Vanatta was always a ready addi- 
tional help. I also express my indebtedness to Dr. Bryant 
Walker, whose generosity made the collection originally pos- 
sible, and whose continued advice has been very helpful. 
Acknowledgment is also made to the many members of the 
staff of the zoological laboratory of the University of Penn- 
sylvania, who have been of assistance in the production of 
the photographs. 
Part I. THe UNIONIDAE OF THE SAN JUAN RIVER SystTEM 
Unfortunately, the water was high, so the only living naides 
obtained were a few small specimens of A. sapotalensis (1,ea) 
and a number of juvenile individuals of a form near A. 
umbrosa (Lea) from the Arroyo Hueyapam (H, vii, b).1. In 
addition, other shells from the Arroyo Hueyapam and La 
Laja (H, vii, a) were picked up on the sand-bars and in the 
streams themselves. 
Fortunately for the collector, some of the natives of the 
region collected the uniones at low water, for the purpose of 
1The Roman numerals and letters, throughout the paper, designate 
the habitats, as classified in the introduction. In order to distinguish 
them from the references to the plates, they are preceded by the letter H. 
