4 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 hy i. de- 
rived much assistance. ee 
IN PHILADELPHIA. 
The Cabinet of Jonn P. WETHERILL. 
» The Cabinet of the Acapremy or NaTuRAL SCIENCES. 
Cc 
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