402 



CHELONIA. 



Pinx 



Qu 



Order 4. PLACODONTIA. 



Known by their skulls only from the Middle Trias (Muschelkalk) of 

 Germany and Russia. With very peculiar dentition of large pavement-like 

 crushing teeth on the jaws and palate, which seem to indicate that they 

 probably lived on hard-shelled molluscs. The skull recalls that of Dicy- 

 nodonts. Placodus Agassiz ; Cyamodus v. Meyer. 



Sub-class 9. CHELONIA.* 



Body encased in a bony capsule, jaws without teeth but with horny 

 beaks, nasal opening single and at the front end of the snout, quad- 

 rate immoveable and appendages with five digits. The lower tem- 

 poral arcade alone is present. Anal opening round or longitudinal. 



No other group of 

 Reptiles is so clearly de- 

 nned and characterised 

 to the same extent by 

 peculiarities of form and 

 organisation as is that 

 of the Chelonia, and 

 this applies to the ex- 

 tinct as well as to the 

 living forms. The in- 

 vestment of the body 

 by a shell composed of 

 an upper, more or less 

 arched osseous plate 

 (carapace], and by a 

 lower ventral plate (pla- 

 stron) forms a character 



as distinctive of the Chelonia, as is the possession of wings and 

 feathers of the class Aves. This shell is covered, except in the 



* A. ~Bo)a,nus, " Anatome Testudinis Europae," Vilnae, 1819. H. Rathke, 

 " Ueb. d. Entwick. der Schildkroten," Braunschweig, 1848. L. Agassiz, 

 " Embryology of the Turtle," Contributions to the Natural History of the 

 United States, 1 and 2, Boston, 1857. A. Strauch, " Chelonologische 

 Studien," Mem. de VAcad. St. Petersbourg, (7) 5, 1862. Gray & Sowerby, 

 Tortoises, Terrapins and Turtles drawn from life, London, 1872. W. 

 K. Parker, " Development of the Green Turtle," Challenger Reports, 1, 

 1880. J. E. Gray, " Notes on the families and genera of Tortoises, and on 

 characters of their skulls," Proc. Zool. Soc. London, 12, 1869. Mitsukuri, 

 Contributions to the embryology of Reptilia, Journal Coll. Sci. Imp. Univ. 

 Japan, 4, 1890 ; 5, 6 ; 1893, 10, 1896. Th. Huxley, " The Anatomy of Verte- 

 brated Animals," London, 1871. G. A. Boulenger, " Catalogue of Chelo- 

 nians in the British Museum," London, 1889. Hoffmann, Gadow, Cope, 

 Zittel, op. cit. 



FIG. 223. Ptychognathus declivis Owen, Karoo Sand- 

 stone. Side view of skull (from Zittel after Owen). 

 ang angular, c maxillary tusk, d dentary, Ju jugal, 

 MX maxilla, N external narial opening, Na nasal, 

 Pa parietal, Pmx premaxilla, Prf prefrontal, Ptf 

 postfrontal, Qu quadrate, Sq squamosal. 



