158 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. III. 



paces have a shagreen or pitted surface, closely resembling 

 the granulated plates under examination ; but in the latter 

 there are deep grooves on the sculptured surface, showing 

 that the original was protected by scutes of tortoise-shell, 

 as in the other tribes of Chelonia. 



"In the rib" (placed on the lowest shelf in Case A. } ) 

 " these imprints are very distinct, and it is observable that 

 this costal plate, instead of being nearly of an equal width 

 throughout, as in the freshwater and marine turtles, gradually 

 enlarges till one termination is twice as wide as the other. 

 This is a character observable in the land tortoises only, and 

 therefore presents another anomaly in the structure of the 

 fossil animal. From the slight degree of convexity of this 

 rib, it is clear that the original was of a flattened form, like 

 the common turtle, Testudo mydas ; its shagreened surface 

 proves .its analogy to Trionyx; but the imprints of scales 

 show that it cannot be identified with any recent species". 

 Among the numerous portions of the osseous border of the 

 carapace found in Tilgate Forest, we have not observed any 

 with a shagreen surface; a negative proof that the fossil, like 

 the recent Trionyx, was destitute of that appendage." 2 



Specimens far more perfect have since been met with; 

 some of which are in the collection of Sir P. Egerton, and 

 described in " Brit. Assoc. Reports." According to the pre- 

 sent state of our knowledge of this remarkable type of 

 freshwater turtles, the carapace was very flat and large, and 

 its surface rugous, as in the Trionyces, but covered with 

 dermal scutes, as in the Emydians : it therefore had not the 

 soft integument of the existing Chelonians, to which it is 

 otherwise nearly related. 



Except in having a defensive coat of tortoise-shell, the 

 Tretosternum Bakewelli, with its sculptured carapace and 

 rudimentary marginal plates, and unossified centre of its 



1 Figured in "Foss. Tilg. Forest," PI. VI. fig. 1. 



2 " Fossils of Tilgate Forest," pp. 60, 61. The palaeontologist en- 

 gaged in establishing the nomenclature of British Fossils, will remark 

 that the above quotation is from a work published in 1827; and that 

 the distinctive characters are sufficiently pointed out to warrant the 

 author in assigning a specific name, six years afterwards (" Geol. S. E. 

 of England," p. 255), to a fossil exhumed from the strata with his own 

 hands, and described and figured in his works alone. 



