28 



CHELONIA. 



shield l . Palate with tall, thin, alveolar walls, and a low ridge on 

 either side, which is continued across the median groove ; pteryg- 

 oids long, not much expanded anteriorly, with the lateral borders 

 posteriorly emarginate, and the ectopterygoid processes, if distinct, 

 placed posteriorly to the antero-external angles ; palatal apertures 

 of temporal fossae antero-posteriorly elongated ; posterior nares in 

 anterior third of cranium ; vomer of moderate length, joining with 

 the short premaxillse. Symphysis of mandible relatively short, 

 convex inferiorly, with the oral surface ridged ; the length of the 

 postsymphysial portion of the rami usually exceeding twice that of 



Fig. 8. 



A 



Oral aspect of the right ramus of the mandible of (A) Chclonemydas and of (B) 

 Lytoloma crassicostatum. a, dentary ; b, coronoid ; c, articular ; d, surface 

 for quadrate ; i, symphysial region ; n, insertion of genio-hyoid muscle. 

 A is #. (After Dollo.) 



1 The shield here termed occipital is equivalent to the interoccipital of Owen 

 (' Reptilia of the London Clay, &c.,' 1849), while the sincipital of that writer is 

 here termed interparietal. 



