LIZARDS. 315 



of the Chelydae. Remarkable instances of discontinuous genera 

 are seen in Hydromedusa, one species of which inhabits the Oriental 

 realm and the remainder the continent of South America, and in 

 Podocnemis, whose species are divided between South America 

 and Australia. 



The earliest chelonian remains occur in deposits of Jurassic age 

 (Switzerland, Germany, France), in which a well-marked differentia- 

 tion of the modern families Emydse Cas seen in the genera Thalas- 

 semys, Eurysternum, Tropidemys, Helemys, the last supposed to 

 have been closely related to the American snapper) and Chelydaa 

 (Plesiochelys, Idiochelys, Craspedochelys) already appears. The 

 number of forms is materially increased in the succeeding Creta- 

 ceous deposits, where, in addition to the representatives of the two 

 families already indicated (e. g., Platemys, Pleurosternum, Adocus, 

 Euclastes, Osteopygis), we have those of the Trionychidas and 

 Cheloniidas (Trionyx, New Jersey ; Chelone, Maestricht chalk, and 

 greensand of New Jersey). Protostega gigas, a marine turtle from 

 the deposits of this age of Kansas, attained a length of upwards of 

 twelve feet. Many of the recent genera, as Testudo, Chelydra, 

 Emys, Cistudo, &c. , appear as fossils in the early or middle Tertiary 

 deposits. The most extraordinary of all extinct forms is the giant 

 land-tortoise of the Siwalik Hills of India, Colossochelys atlas, 

 which measured apparently not less than fifteen to twenty feet in 

 length. Of somewhat less than one-half these dimensions was the 

 Macrochelys mira, from the molasse of Southern Germany (Ober- 

 kirchberg, naar Ulm), whose modern representative is the Missis- 

 sippi snapper (Macrochelys lacertina). 



Lacertilia. The number of known species of. lizard is esti- 

 mated by Giinther to be about seventeen hundred, of which by far 

 the largest part is confined to the warmer regions of the earth's 

 surface. But comparatively few forms are found to pass beyond 

 the fortieth parallel of latitude, and at about the sixtieth parallel 

 (north) the order practically disappears. The most northerly spe- 

 cies is Lacerta vivipara, whose range comprises nearly the whole of 

 Europe, and extends northward to the seventieth parallel (in Nor- 

 way) ; it is accompanied as far as Lapland by the no less broadly 

 distributed blind- worm (Anguis fragilis). In the Western Hemi- 

 sphere the northward extension of the order is much more limited 

 than in the Eastern, and it would appear that only one species, a 



