40 
INTRODUCTION. 
in the branch of knowledge to which he had devoted 
himself, which has forever associated his name with the 
history of Zoology in the United States, and which ought 
to entitle him to the honor of being styled its founder. 
His published works, commencing about the year 1817, 
continue through a period of eighteen years until his 
death in 1834 ; and embrace descriptions of a vast num- 
ber of animals until then entirely unknown, or but 
imperfectly understood. The invertebrated classes 
received the greater part of liis attention, and his con- 
tributions to their history must, necessarily, form an 
important portion of the means of any writer who shall 
seek more fully to elucidate them. The major part of 
the subjects described in this volume were first described 
by him ; and so fully had he occupied the ground, that 
the additions to our knowledge of species made during 
the twelve years, since his decease, have not exceeded 
the number contributed by him, although within that 
time many new inquirers have commenced their investi- 
gations, and large districts of country, which he never 
examined, have been explored. His descriptions are for 
the most part so accurate, and his observations so just, 
that there is little left to his successor except to bring 
them together in their proper order. Some obscurity, 
indeed, attends several of his species, in consequence of 
his not having uniformly placed specimens in a public 
depository for reference ; for, when objects resemble each 
other so closely as do several species of the genera Helix 
and Pupa, it is very difficult to distinguish them by 
