AND AVES OF NORTH AMERICA. 



201 



readily identified, and its author lias given the name the position of a synonyme. Owens' 

 Liodon presents teeth similar to those of M. impar, but here again we do not know any 

 other characters of the genus. 



The characters are well distinguished from those of Clidastcs on the one hand, and 

 Mosasaurus on the other. The vertebrae are those of the latter genus, while the ptery- 

 goids do not present that union on the median line characteristic of the same. They have 

 not the horizontal expansion scum in Clidastes. Their teeth are not arranged after a type 

 so strongly pleurodont as in Platecarpus, but in the type species they are more exposed 

 than in Mosasaurus giganteus, M. missuriensis and M. dekayi. As Leidy describes the 

 pterygoid bone of Platecarpus tympaniticus, T am induced to suppose that they are united 

 on the median line; the width above is nearly one-third the length, and a. shelf " projects 

 on the inner side separating the upper from the lower surface." This is as in Mosasaurus. 

 and constitutes probably, a point of union of the two hones. 



This separation of the Pterygoids is characteristic of Lacertilia and Ophidia generally, 

 while the continuous relation belongs rather to Crocodilia, Sauropterygia, etc. 



The entire distinctness of the chevron hones of the caudal vertebra; is remarkable as 

 differing from the known structure of M. giganteus, M. missuriensis, M. dekayi, and Cli- 

 dastes propython. The large size of the L. proriger renders it improbable that this fea- 

 ture is to he ascribed to immaturity, especially as the two species last named as differino- 

 in that respect, arc; relatively quite small. A species from the rotten limestone of Ala- 

 bama, presents a similar characteristic of the caudal vertebra, hut whether its pterygoids 

 be those of Liodon or Mosasaurus, 1 am unable to ascertain. 



Pterygoids nine, smaller, cylindric ; mandibulars eleven, much larger; anterior teeth 

 convex inwardly, indistinctly fluted; premaxilla with two teeth on each side, prolonged 

 into a, cylindric rostrum. L p R0RIGER 



Pterygoids eight, unequal, the median large, almost equal to tin; mandibulars; teeth 

 compressed, with two edges, inner sides little convex ; no flutes or marked facets. 



L. MITCHILLII. 



The only character by which I "distinguish the L. validus from Mosasaurus, is the pe- 

 culiar form of the os quadratum. The L. kiwis is characterized by the form of its verte- 

 bra 1 , which differs considerably from that of any species of Mosasaurus. They may be 

 compared as follows: 



Cervical cups vertically oval, proximal caudal subhexagonal ; surfaces striate; size 

 ' iU 'tf°- L. PRORIGER. 



Articular faces cervicals and anterior dorsals round; posterior dorsal depressed centra 

 short, broad; large. L. VALIDUS. 



AMEBIOA.. I'lIU.O. SOC. — VOL XIV. 51 



