PEEFACE. 



The Collection which forms the subject of this Catalogue is 

 particularly rich in Chelonians from the Purbeck beds of Swanage, 

 the Cretaceous of England and Holland, the Eocene Tertiaries of 

 Harwich, Sheppey, Hampshire, the Isle of Wight, and the older 

 Pliocene of the Siwaliks of India ; the last-named beds have also 

 yielded the largest tortoise known (the Testudo \_Colossochelys] atlas 

 of Ealconer). 



The last-surviving species of Chelonian indigenous to England 

 was the Emys orbicularis, Linn., whose remains have been found in 

 fluviatile (Post-Pliocene) deposits in Norfolk (see Geol. Mag. 1879, 

 dec. ii. vol. vi. pi. viii. p. 304). Chelonians were well represented 

 here in Tertiary and Cretaceous times ; some of the largest forms 

 (as Chelone hoffmanni and EospJiargis gigas) occurring in the Chalk 

 and London Clay. 



Of the fifty-two genera and one hundred and thirty-one species, 

 or varieties, of fossil Chelonians recorded by Mr. Lydekker, only 

 eighteen genera and ten species can be with certainty correlated 

 with living forms. 



The number of extinct species is no doubt in some cases too large, 

 owing to the difficulties which surround the task of attempting 

 decisively to determine the fragmentary and frequently headless 

 remains of Chelonia met with in a fossil state. 



