1. TllIONYX. 245 



2. A single neural i)late between the first pair of costals ; 



alveolar surface of lower jaw with a strong- luiigitudinul 

 symphysial ridge. 



Epiplastra widely separated from each 



other; plastral callosities well deve- 

 loped and largely pitted; width of the 



postorbital arch, in the adult, more 



than one third the diameter of the 



orbit ; young with four large occUar 



dorsal spots 5, formosus, p. 250. 



Ei)iplastra in contact in front of the 



entoplastron ; plastral callosities very 



feebly developed, finely sculptured ; 



width of the postorbital arch, in the 



adult, more than one third the dia- 

 meter of the orbit G. phayrii, p. 251. 



Epiplastra in contact in front of the 



entoplastron ; plastral callosities well 



developed and largely pitted ; width of 



the postorbital arch not one third the 



diameter of the orbit 7. cartilagineus, p. 253. 



3. A single neural plate between the first pair of costals ; 



alveolar surface of lower jaw without a longitudinal 

 symphysial ridge. 



a. Last pair of costal plates well developed ; plastral callo- 

 sities well developed in the adult : snout (on the skidl) 

 considerably longer than the diameter of the orbit. 



Entoplastron forming a right angle .... 8. tnimgids, p. 254. 



Entoplastron forming an obtuse angle . . 9. sinensis, p. 256. 



h. Last costal plate not more than half the size of the 

 penultimate ; plastral callosities very feebly developed, 

 absent on the xiphiplastra ; snout (on the skull) not 

 longer than the diameter of the orbit ; proboscis very 

 short. 



Entoj^lastron forming a right angle .... 10. sivinJionis, p. 257. 



Entoplastron forming an acute angle . . 11. euphraticus, p. 258. 



II. Costal plates seven pairs normally ; young with smooth or 

 finely granulate dorsal skin. — Korih America* . 



Snout (on the skull) obtuse, hardly as 

 long as the diameter of the orbit ; 



* The following species require further investigation : — 



1. Aspidonectcs asper, Agass. Contr. N. H. U. S. i. p. 405 (18.57). — 



Mississippi. 



2. Aspidonectcs ntichalis, Agass. /. c. p. -JOG. — Cumberland and Tennessee 



Rivers. 



I regard the species named by Eafinesque, Atlant. Journ. p. 64 (1832), 

 Apalone htidsonica and Mesodeca bartraTid, as mythical. 



s2 



