PREFACE. I'll 



might not meet with a sale, proportioned to the expense of 

 undertaking ; he therefore considered it to be necessary to adapt it, 

 as much as possible, to readers in general. Another consideration 

 also influenced him to this he thereby hoped to lead those to 

 the study of this part of Nature's works, to whom, as yet, they 

 might be unknown. The epistolary form, from the familiarity of 

 style which it admits, was therefore adopted ; especially since it 

 yielded the opportunity of introducing such a portion of intro- 

 ductory matter, as might secure the comprehending of the discus- 

 sion by those, to whom it might be, in a great measure, new. In 

 doing this he was, however, aware that he was trespassing on those 

 who were already informed on thesfe Subjects ; but, reckoning on 

 the complacency and liberality with which men of true learning 

 promote the diffusion of knowledge, he considered their excuse as 

 already obtained, and no longer viewed this as an objection. - 

 Gratitude directs, that he should declare how much he is himself 

 indebted to the prevalence of this generous disposition. With 

 grateful remembrance of the kindnesses he received from the late Dr. 

 Gray, Secretary of the Royal Society, he acknowledges his ready 

 assistance on many occasions, as well as the kind facilities he in- 

 dulged him with, during his examinations of the numerous valuable 

 specimens, contained in that department of the British Museum 

 which he so ably superintended. To Dr. Shaw he is also obliged 

 for similar attentions. To Charles Hawkins, Esq., William Long, 

 Esq., George Chandler, Esq., Sir Charles Blick, John Heaviside, 

 Esq., Sir William Blizard, and Henry Cline, Esq., the Curators of 

 the Hunterian Museum ; he is under great obligations for like 



