ANT) AVES OF NORTH AMERICA. 



1 85 



group, and indeed in any "roup where they are characterized by such simplicity of form, 

 renders it probable that there will be found a complete gradation between different genera 

 in this respect. 



I. Cervical vertebrae with separated articulating by] (apophyses. 

 Pterygoid bones united on the middle line. 

 The pterygoid teeth pleurodont. 

 The pterygoid teeth in alveoli. 

 Pterygoid bones vertical, separated throughout their length. 



II. Cervical vertebra? with simple continuous hypapophyses. 



PLATECARPUS. 



MOSASATJRTJS. 



LIODON. 



BAPTOSAURUS. 



WO ff( 



mera have been defined from dental characters, as follows : 



Crowns of the teeth curved, with one face smaller than the other, more or less atten- 



uated and acute; numerous narrow ridges on inner face. 



1IOLCODUS. 



Crowns of the teeth straight, compressed, more slender, regularly acuminate ; faces 



equal, facets distinct, numerous. 



POLYGONODON. 



The genus Diplotomodon, Leidy, known from a single; very much compressed dental 

 crown with minutely denticulated edges, may belong here. 



To what extent and in what manner the dental characters of those genera established 

 on them, coincide with those distinguishing the genera of the table, it is not now easy to 

 discover. In species referable to Maorosaurus, teeth of the compressed form and lenticu- 

 lar section characteristic of Liodon, are found in one species, while another presents the 

 more angular facetted section of Mosasaurus in the anterior portion of the series. Both 

 forms seem to occur in Mosasaurus proper, according to Leidy. Of these genera, founded 

 on dental forms, I'olygonodon, Leidy, is the best marked, and will prove distinct from any 

 of the above. 



The. humerus of a verf large reptile in the Mus. Acad., Phil., has been regarded by 

 Agassi/, as belonging to a Chelonioid, and by Leidy been referred to the common Mosa- 

 saurus of New Jersey. There is at present no means of assigning it to any species of tins 

 order otherwise defined. It is the Atlantochelys mortoni Agass., and may refer to any large 

 species of the genus so far as our knowledge goes ; it was not originally described, however, 

 and cannot therefore retain this name. The Mosasaurus dehayi, Broun, is founded on a 

 tooth exactly like that of M. mitchillii of Leidy, and cannot be distinguished on such basis 

 alone;. The M. impar is only known from jaws and teeth, and hence is the only species 

 whose name is liable to have; been duplicated here. It may belong to any of the Ameri- 

 can species of liodon here enumerated, except L. validus and L, proriger, whose 



AMEEIOA. I'll 1 1, t). sod. — VOL. XIV. 47 



