P H K FAC E. 



With certain slight exceptions, which we have pointed oul in the text, the 

 following essay is in the strictest possible sense the joint production of its two 

 authors, who are equally responsible for all of its statements. 



The woodcuts owe much of their accuracy to the skill of the engraver, Mr. 

 Wilhelm, to whose experience as an anatomical draughtsman the authors are under 

 obligation. 



They entertain the wish that the novel views of the present paper may induce 

 comparative anatomists and physiologists to examine afresh the respirator) mechan- 

 ism of other reptiles, and also of birds— a labor in which they indulge the hope of 

 sharing. 



S WEIB MITCHELL, 



>'-... 1228 Walnut Street, Philadelphia 



GEO. R. MOREHOUSE, 



No 2-27 South Ninth Street, Philadelp 



