86 TORTOISES AND TURTLES. 



This is effected by immersing it in hot water, and then allowing it to cool under 

 heavy pressure between smooth blocks of wood, or metallic plates. The surface is 

 then rendered smooth, and the thickness equal, by scraping and tiling away the 

 rough and prominent parts. In this way each plate receives an equal and smooth 

 surface. But it is in many cases desirable to employ larger pieces than can be 

 obtained from single plates, and two pieces are then united together in the 

 following manner. The edges are bevelled off to the space of two or three lines, 

 and the margins, when placed together, overlap each other to that extent. They 

 are then pressed together by a metallic press, and the whole is submitted to the 

 action of boiling water; and by this means the two pieces are so admirably 

 soldered together as to leave no indication of the line of union. By the application 

 of heat, also, the tortoise-shell may be made to receive any impression by being 

 pressed between metallic moulds." Necklaces, etc., are made by pressing the 

 fragments and dust in moulds. 



Turtles, more or less closely allied to the existing kinds, abound 

 Extinct Turtles. J 



in marine strata of the Tertiary and Cretaceous epochs, some belong- 

 ing to extinct and others to the living genera. Among the latter, the gigantic 

 Hoffmann's turtle (Chelone hoffmanni), from the chalk of Holland, appears 

 to have been allied to the hawksbill, but had a shell of some 5 feet in length. 

 Extinct loggerheads occur in the London Clay; and an allied extinct genus 

 (Lytoloma), common to the same formation and the upper Cretaceous deposits, was 

 remarkable for the great length of the bony union between the two branches of 

 the lower jaw, and also for the circumstance that the aperture of the internal 

 nostrils was placed right at the hinder extremity of the palate, as in crocodiles. 

 In strata older than the Chalk, such as the Purbeck and other Oolitic rocks, we 

 meet with turtles having heart-shaped shells, but clawed limbs, and a vacuity in 

 the centre of the plastron, these forming an extinct family (Acichelyidce), from 

 which the modern turtles have probably originated. 



LEATHERY TURTLES. 



Family DERMOCHELYID^. 



The remarkable leathery turtle, or luth (Dermochelys coriacea), which is the 

 solitary survivor of a series of extinct forms, is one of those animals whose serial 

 position is a matter of dispute among naturalists ; some of whom regard it as so 

 different from all other Chelonians, that it ought to represent a suborder by itself, 

 while others believe it to be merely a highly specialised form allied to the true 

 turtles. From the evidence afforded by extinct species, the latter view, to our 

 thinking, appears the more likely to be the true one. The essential peculiarity of 

 the leathery turtle is to be found in the nature of its carapace, which is a mosaic- 

 like structure composed of a number of irregular discs of bone closely joined 

 together, and entirely free from the backbone and ribs. In certain extinct forms 

 the carapace, on the other hand, is represented merely by a row of marginal bones ; 

 from which it is inferred that these reptiles have been derived from true turtles by 

 a gradual disintegration and breaking up of the carapace. In the living genus the 



