25 



ACKNOWLEDGMENTS. 



Every author who attempts a Monograph of any 

 of the departments of Natural History, must neces- 

 sarily depend, in a greater or less degree, upon the 

 kindness and liberality of others. Rare and unique 

 specimens, particularly of fossil species, are often 

 scattered through different cabinets, and his work 

 would be rendered very imperfect, if they were not 

 intrusted to his care. In preparing the following 

 Monograph on the plan of giving exact models of 

 the species, instead of illustrating them by engrav- 

 ings in the usual manner, the specimens when used 

 by the artist are perhaps more liable to accident, and 

 it was at first supposed that this circumstance might 

 have prevented the original design. But in no in- 

 stance, where an application has been made, either to 

 a public institution or to a private cabinet, has the au- 

 thor met with a refusal; indeed the courtesy, kind- 

 ness, and liberality which he has experienced from 

 naturalists, who have every where aided him in the 

 prosecution of his work, form no inconsiderable 

 portion of the gratification which he has received. 

 Besides the acknowledgments to public museums, 

 and to individuals, which are made in the body of 

 the work, the author is desirous of recording in this 

 place, the following cabinets from which he has de- 

 rived much assistance. 



IN PHILADELPHIA. 



The Cabinet of JOHN P. WETHERILL. 



The Cabinet of the ACADEMY OF NATURAL SCIENCES. 



