AND AYES OF NORTH AMERICA. 



133 



Fiff. 38. 



OSTEOPYGIS PLATXLOMTJS carapace. 



The xiphisternal is united with the hyposternal precisely as in Chelydra, that is, 

 by a mutual marginal gomphosis at the anterior extremity of the former, its peg being 

 the external; and by a deep groove in the inner margin of the xiphisternal, which 

 receives the adapted margin of the hyposternal. The xiphisternal lobe is wider than in 

 Chelydra, while the bridge is very much wider, being about as in Trionyx, and much 

 narrower than in Chelone. This constitutes a point of separation from the first named 

 genus. There is a median fontanelle, larger than in Chelydra serpentina at the junc- 

 tion of the four sternal elements. The external margin of the hyosternal, is free and 

 serrate as in Chelydra. The mcsosternal has not been identified, but the epistcrna are 

 narrower and greatly incurved, more as in Chelydra and Chelone than as in Trionyx. 

 In the accompanying restoration, the shaded portions are those which have not been pre- 



AMKRI. PHILOSO. SOO. — VOL. XIV. 34 



