Occasional Papers of the Museuin 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 ever)' 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 System 



Unfortunately, the water was high, so the only living naides 

 obtained were a few small specimens of A. sapotalensis (I,ea) 

 and a number of juvenile individuals of a form near A. 

 nmhrosa (Lea) from the Arroyo Hueyapam (H, vii, b).^ 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 



^ The 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. 



