AND AVES OF NORTH AMERICA. 29 



PYTHONOMORPHA. 



Characters of skeleton as the preceding, except : opisthotic distinct, prolonged from the 



cranial walls as suspensorium of the quadrature. 

 Xo symphysis mandibuli. 

 Squamosal present ; premaxilliary single. 



Cranium with alisphenoid and parietal developed in front of prootic. 

 Limbs inflexible, natatory ; sacrum none ; chevron bones. 

 Neural arches not attached to centrum by suture. 

 Pubes and ischia wanting. (!) 



OPHIDIA. 



Characters of Lacertilia except : opisthotic free, distinct from the cranium except proxi- 

 mally, supporting quadratum ; no squamosal. 



Cranial cavity largely ossified anteriorly. 



Palatine bones free from other elements except pterygoids. 



No symphysis mandibuli. 



Sternal and pelvic arches wanting ; no limbs except rarely rudiments posteriorly ; no 

 sacrum. 



Vertebra? united by double articulation ; neural arches continuous with centra ; no chev- 

 ron bones. 



Circulatory and Nervous Systems, in important features as Lacertilia. 



ICHTHYOPTERYGIA. 



ICHTHYOSAURUS, Conybeare. 



Leidy says with reference to the species here described, " They have an affinity to 

 Ichthyosaurus and Eosaurus, nor am I prepared to prove that they do not belong to one 

 of these." 



ICHTHYOSAURUS GRANDIS, Leidy. 

 C7ionespondylus yrandis, Leidy. Proc. Acad. Nat. Sci., Philada., 1868 — 178. 



Humboldt Co., Nevada. 



AMERI. PHILOSO. SOC. — VOL. XIV. 8 



