TRIONYX. 11 



a'. Inner edge of alveolar surface of mandible 

 strongly raised, forming a sharp ridge, 

 which sends off a short perpendicular 

 process at the symphysis ; interorbital 

 space, in the adult skull, narrower than 

 the external nasal opening ; head with 

 oblique black lines, converging for- 

 wards ; no ocellar spots T. yangcticus, p. 12. 



I'. Inner edge of alveolar surface of mandible 

 not or but very slightly raised, sym- 

 physis flat or with a longitudinal furrow ; 

 interorbital space narrower than the 

 external nasal opening; head with 

 oblique black lines, converging for- 

 wards ; young with small ocellar dorsal 

 spots T. leithii, p. 12. 



c'. Inner edge of alveolar surface of mandible 

 not or but very slightly raised, sym- 

 physis flat or with a longitudinal fur- 

 row ; interorbital space, in the adult 

 skull, broader than the external nasal 

 opening; head black-marbled, with a 

 large yellow spot behind each eye; 

 young with large ocellar dorsal spots . . T. hurum, p. 13. 

 b. A siiigle neural plate between the first pair 

 of costals; alveolar surface of lower jaw 

 with a strong longitudinal symphysial 

 ridge. 



a'. Epiplastra widely separated from each 

 other; plastral callosities well deve- 

 loped and largely pitted T.formosus, p. 14. 



b'. Epiplastra in contact in front of the ento- 

 plastron ; plastral callosities very feebly 

 developed, finely sculptured T. phayrii, p. 14. 



c'. Epiplastra in contact in front of the ento- 

 plastron ; plastral callosities well deve- 

 loped and largely pitted T. cartilagineits^, 15. 



4. Trionyx suhplanus. 



Trionyx subplanus, Geoffr. Ann. Mus. xiv, 1809, p. 11, pi. v, fig. 2 ; 



Qiinth. Kept. B. I. p. 49 ; Bouleng. Cat. Chel. $c. p. 246. 

 Dogania giintheri, Gray, P. Z. S. 1802, p. 265. 

 Trionyx giintheri, Gilnth. Rept. B. 1. p. 49, pi. vi, fig. A. 



Carapace very flat ; costal plates eight pairs, the last well deve- 

 loped and separated by the last neural plate ; a single neural 

 between the first pair of costals ; these plates very finely granulate 

 and vermiculate ; a large fontanelle, till late in life, between the 

 nuchal and dorsal plates. Epiplastra narrowly separated from 

 each other in front of the entoplastron, which forms an obtuse or 

 a right angle ; plastral rugosities scarcely developed, on the hyo- 

 and hypoplastra and on the xiphiplastra. Dorsal skin of young 

 with interrupted longitudinal ridges. Head very large ; snout (on 

 the skull) about as long as the diameter of the orbit, which is more 

 than double the interorbital width; postorbital arch extremely 



