1913.] NATURAL SCIENCES OF PHILADELPHIA. 619 



the river into Bolivia on three occasions. I wish here to extend 

 my most sincere thanks to Messrs. May, Jekyll & Randolph for their 

 courtesy, which made the work done on this part of the trip possible, 

 and to the many employes of the Madeira-Mamore Co. who rendered 

 us assistance and personal courtesies in very large measure during 

 this most delightful journey. 



By a curious difference in the time of the dry season of the different 

 districts visited, all the work of the Expedition was done during the 

 dry season in each district, except that the rainy season was just 

 beginning as we left the Madeira River and during our last short 

 stay in Para. Everywhere we found shells sestivating, and it is 

 probable that this accounts for the somewhat limited list of species 

 actually taken by the members of the Expedition and for the large 

 number of dead shells which made up much of the collection. 



The ground covered by the Expedition and by the locations from 

 which the mollusks here reviewed wer^ taken can be divided easily 

 and naturally into two distinct districts, the second of which, on 

 account of the great extent of the country included and because a 

 distance of nearly five hundred miles intervenes between the two 

 nearest locations, can be again divided conveniently into two dis- 

 tricts. The first, northeastern Brazil, includes about all of the 

 States of Rio Grande do Norte and Ceara; it is scantily timbered 

 as a rule, dry, with an average rainfall of from ten to twenty inches, 

 and a climate not unlike that of Southern California, Arizona, and 

 New Mexico. The coast region is generallj^ low, with a few low ranges 

 of mountains, the interior higher. In this district are included 

 Natal, Pirangy, Papary, Estremoz, Ceara-Mirim, Taipii, Baixa 

 Verde, Limoeira, and the Mossoro region, Ceara, Maranguape ]Moun- 

 tain, Mongiiba, Buturite, Quixada, and Camocim. 



The other main district, with Maranhao on the outskirts, includes 

 all other locations mentioned in this paper, beginning with Para 

 and ascending the Amazon and its tributaries, the Tocantins, Jary, 

 Jamauchim, Maccurii, Tapajoz, Madeira, Mamore, and various other 

 small streams. This district is an immense, low, alluvial plain, 

 almost universally heavily timbered, in most of which there is a long 

 rainy season, the annual rainfall averaging up towards a hundred 

 inches in many years, filling many streams more than bank full and 

 producing great sloughs and lakes. Naturally, the climates of the 

 two districts are radically different, although a considerable number 

 of species overlap. As noted above, it is convenient to divide this 

 district into two at any point between the mouth of the Tapajoz 



