PREFACE. 
IN an early stage of my inquiries into the structure of that interesting 
class of Animals which forms the subject of the present treatise, I had 
it in contemplation to limit them to the space of a brief memoir, which 
I had intended to submit to the Linnean Society ; as, however, these 
inquiries proceeded, aided by the acquisition of numerous new and 
important specimens, and by the kind contributions of my scientific 
friends, they gradually swelled to such a bulk (and more particu- 
larly the plates requisite for their illustration increased so greatly in 
number) that I could no longer flatter myself that a Society whose 
objects are so general, would deem it expedient to sacrifice so large a 
space in its transactions to a single branch of Natural History, and 
still less to incur on such an account, so great an expense as regular 
engravings of such numerous details would necessarily have entailed. 
Under these circumstances, many of the friends to whose inspec- 
tion 1 had submitted my collection of these remains, and whose 
acquaintance with the subject entitled their opinion to an high degree 
of deference, were kindly anxious that I should still communicate ` 
the result of my investigations to the Public, by offering on the terms 
of Subscription, a ras of this Class to which they related, 
and applying the slender talent of drawing I possessed, to executing - 
myself the necessary illustrations in a series of Lithographical Plates, 
as the only mode in which a private individual could bring forward 
