2 PREFACE. 



would correspond to the great geological divisions or systems now in general use, 

 viz. : — 



1. Tertiary. 4. Permian. 



2. Cretaceous. 5. Carboniferous. 



3. Oolitic. 6. Devonian. 



7. Silurian. 



So that each Part would form a complete Monograph zoologically arranged as far as the 

 state of our present knowledge will admit.^ 



It has also been considered desirable to preface the main subject of this work by some 

 preliminary observations on the class in general ; in order to enable the reader better to 

 understand why certain sections and names have been rejected, or admitted in preference 

 to others ; and it has afforded me no small pleasure to find that Professor Owen and Dr. 

 Carpenter were likewise disposed to annex their recent and most important anatomical 

 and structural investigations to the general Introduction I was then preparing. And, in 

 accepting so liberal an offer from those eminent and justly celebrated zoologists, I felt 

 convinced that I was attending not only to the best interests of Science, but also to 

 those of the Society, — interests which it is the main object and desire of all the members 

 to advance. 



THOS. DAVIDSON. 



London ; January 4, 1853. 



great number of the localities themselves : and, from having been at the commencement more prepared with 

 the Oolitic species, I deemed it advisable to begin with them, especially, as by so doing, no subsequent 

 inconvenience would be incurred by the members of the Society. I must also avail myself of this 

 opportunity to express my warmest thanks to all those local geologists who have so kindly and liberally 

 assisted in the prosecution of my investigations ; many of these gentlemen I have already had the pleasure 

 of naming in portions of the work already published, and, in addition, I have now the satisfaction of 

 mentioning Professor Owen, Dr. Carpenter, Drs. Gray, Volborth, and Perier ; Professors Beyrich and 

 Kutorga ; Col. Helmersen ; Messrs. Salter, Cuming, Hanley, de Loriere, de Hagenow, and Michelotti ; and 

 I am most particularly indebted to Professor King, Mr. Woodward, and M. Sucss, of Vienna, for much 

 valuable information, and for the kind interest they have shown in the progress of tiie woi'k. 



^ It has been estimated that the work now in progress will require from eighty to one hundred plates, 

 and it was found desirable to divide the subject into two volumes, the first will be devoted to the Tertiary, 

 Cretaceous, and Oolitic species ; the second, to the Palceozoic forms. 



