PREFACE. 
Tue following Memoir was commenced seven years ago, and, although various 
circumstances have interfered with its completion until now, the delay has not 
been unattended with some advantages. During the lapse of time nearly as much 
new material has been discovered as was originally at the command of the author, 
and thus our acquaintance has been greatly extended with the subjects of the 
memoir. In consequence the latter has been repeatedly altered, and portions have 
been intercalated, which may serve to explain any apparent want of continuity in 
the work. 
The author takes the opportunity of acknowledging his obligations to Prof. 
George H. Cook, of Rutgers College, New Brunswick, New Jersey, who has 
given constant and important aid in procuring specimens for examination dur- 
ing his geological explorations of the State. Acknowledgments are also due 
to W. Parker Foulke, of Philadelphia, through whose especial exertions we 
are indebted for the discovery of the huge Reptile, Hadrosaurus, which forms 
one of the most interesting and important subjects of description in the suc- 
ceeding pages. Valuable assistance has also been rendered by others in ob- 
taining specimens for investigation, but more especially by Dr. J. H. Slack, of 
Philadelphia. The basis of the work has been mainly founded on the rich collee- 
tion of the Academy of Natural Sciences of Philadelphia. 
( iii ) 
