1. TRIONYX. QA5 
2. A single neural plate between the first pair of costals ; 
alveolar surface of lower jaw with a strong longitudinal 
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 ocellar 
dorsalispotsr “50h suse ccs sneaks os 5. formosus, p. 250. 
Epiplastra in contact in front of the 
entoplastron ; plastral callosities very 
feecbly developed, finely sculptured ; 
width of the postorbital arch, in the 
adult, more than one third the dia- 
MeterOR TNE OLDIG sara see ree ai 6. 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 theiorbit «0.2.55. 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 skull) 
considerably longer than the diameter of the orbit. 
Entoplastron forming a right angle .... 8. triwnguis, p. 254. 
Entoplastron forming an obtuse angle .. 9. simensis, p. 256. 
6. 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. 
Entoplastron forming a right angle .... 10. swinhonis, 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.— North America*. 
Snout (on the skull) obtuse, hardly as 
long as the diameter of the orbit; 
* The following species require further investigation :— 
1. Aspidonectes asper, Agass. Contr. N. H. U. 8S. i. p. 405 (1857)— 
Mississippi. 
2. Aspidonectes nuchalis, Agass. l. c. p. 406.—Cumberland and Tennessee 
Rivers. 
I regard the species named by Rafinesque, Atlant. Journ. p. 64 (1882), 
Apalone hudsonica and Mesodeca bartram?, as mythical. 
82 
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