INTRODUCTION. 



presents advantages which no other possesses. While it has 57,500 square.s miles of surface, it is 

 intersected throughout with large rivers, and an immense number of small streams, which generally 

 have their sources in the high lands, far in the interior, and drain the surface in three directions, 

 viz: — into the Tennessee River to the North, the Gulf of Mexico to the South, and the Atlantic to 

 the South-East. 



During the past two years, many zoological friends in various States, anxious to promote a 

 knowledge of the structure of the soft parts, which we have all been too much in the habit of non- 

 observance, have most obligingly aided me, by sending living specimens, in some cases more than 

 500 miles, which has enabled me to examine them under most favorable circumstances. Others, 

 at greater distances, have sent them in alcohol. With those soft parts now described in this 

 volume, and those descriptions which I have carefully made of previously known species, their 

 number exceeds 200, and as I am constantly in receipt of others, I hope, in time, that most of the 

 species in the United States will be known, as to their anatomical differences. 



The embryonic forms of 38 species in this volume, have been well illustrated in the plate. This 

 Is a very interesting part of the development of this family ; the difference are most striking, and a 

 new field of observation is opened in regard to them. This subject will be continued in the next 

 volume ; already some drawings are made of new forms. 



Since the publication of vol. 5th, I have published some observations on the hjssus* of the 

 Unio, as well also on the visual organsf of some of the species ; since then I have remarked that 

 U. cylindricus, U. rubiginosus, and An. imbecilis, are sensitive to light, in addition to those pre- 

 viously recorded. 



* Proc. Acad. Nat. Sci., Sept. 1856. f P™c. Acad. Nat. Sci., Feb. 1857. 



