INTRODUCTION, 
In striking contrast with what has occurred in other groups of lower 
Vertebrates, the increase of known species of Chelonians has been 
very slight within the last twenty years. In fact, the present 
volume records a considerably smaller number of species than did the 
last-published treatise, viz. Dr. Gray’s ‘‘ Supplement to the Catalogue 
of Shield Reptiles,” in 1870. This decrease is of course due to the 
different views held by the writers as to what deserve to constitute 
specific characters, and also, I trust, to a better understanding, at 
the present day, of the amount of variation within given forms, 
But the fact that hardly 20 valid species have been discovered since 
the close of Dr. Gray’s labours shows that our knowledge of the 
existing Chelonians is far nearer completion than that of any othcr 
group of Reptiles, Crocodiles excepted. 
Though deficient in noyelties,—not a single new species or genus 
is established,—I trust that the present account will prove of interest 
for the thorough revision which higher and lower groups have 
undergone, and for the information it contains on the osteological 
characters, which have hitherto been much neglected. Figures of 
the skulls and shells (stripped of the epidermal shields, and show- 
ing the arrangement of the latter as well as of the underlying bony 
plates *) are given for the principal genera. Although imperfect 
as regards the sutures and other details, the woodcuts of the skulls 
given by Dr. Gray in some of his papers are still useful to show 
the shape and general characters of the skulls, and therefore have 
been reproduced in the present volume. As to the original figures, 
which have been executed with great care, I hope they will prove 
welcome to paleontologists as well as to zoologists. 
* Crosses have been inserted in these figures to indicate the position of the 
axillary and inguinal buttresses of the plastron, which anchylose with the lower 
surface of the carapace. 
