120 Baron Francis Nopcsa, Jim. — The Origin of Mosasaurs. 



Carsosaurus Kornh., Opetiosaurus Kornh., and perhaps also Meso- 

 lepfos Cornaglia.^ 



The following chief differences are observable between these 

 two types : — 



DOLICHOSAURS. 



Skull relatively small. 

 Quadrate bone probably slender. 

 Mandible slender. 

 Teeth thecodont ? 



Vertebrce : neck remarkably long, con- 

 sisting of 13 vertebrse. 

 26 dorsal vertebrae. 



Eibs : dorsal ribs equally long and rela- 

 tively short. 

 No ventral ribs. 



Limbs : anterior limbs much reduced in 



size. 

 Hind-limbs twice as long as fore-Kmbs. 

 Metatarsal v not showing Varanid 



modification. 



AlGIALOSAUKS. 



Skull relatively large. 

 Quadrate round as in Mosasaurs. 

 Mandible strong. 

 Teeth in sockets. 



Neck short, composed of only 7 vertebrae. 



21 dorsal vertebrae. 

 The long dorsal ribs vary in length. 



Ventral ribs well developed. 

 Anterior limbs comparatively long. 



Only somewhat longer. 

 Metatarsal v showing distinctly Vara- 

 nid modification. 



According to the description here given the Dolichosaurus, as pointed 

 out by Osborn, cannot be the ancestor of the Mosasaurs, whereas 

 the same cannot be said of the Aigialosaurs. The most remarkable 

 points of resemblance and difference between the Aigialosaurs and 

 Mosasaurs are as follows : — 



Aigialosaurs. 



MOSASAUKS. 



Skull : tipper surface in both cases perfectly the same and strongly resembling the 

 Varauids. Quadrate bone in both types broad and flattened, thus differing from the 

 same bone in the living Varanidse. Articulation between the front and hind parts of 

 the mandible present in both types and absent in the Varanidse. 



Vertebrce: 7 cervical vertebrae, no sign 

 of primitive structure ; 21 dorsal 

 vertebrae, like those of the Varanida^. 



Seven cervical vertebrae, showing very 

 primitive structure (due to pelagic 

 life ?) ; dorsal vertebrae varying from 

 22 {Tylosaiirus) to 36 {Clidastes). 



Ribs : the sternal ribs are equally developed in both, and their position indicates 

 in both cases the same kind of sternum, with this diiference: — 



Sternum comparatively small ; 10 true 



sternal ribs present. 

 Shoulder- ojirdle somewhat reduced. 



Sternum broad and large ; 6 sternal ribs 



touching the sternum. 

 Shoulder -girdle well developed. 



Fore-limbs intermediate between the Fore-limbs changed to paddles 

 Mosasamid and Varanid type. 



Pelvic girdle well developed. 

 Two sacral vertebrae present. 



Sind-limbs like front-limbs, both ex- 

 tremities bearing sharp claws. 



Pelvic girdle reduced. 

 Only one sacral vertebra. 



Hind-limbs paddles ; hyperphalangy, and 

 no claws present. 



Dermal covering consisting in both cases of rhomboidal scutes bearing a sHght 

 median elevation. 



It is thus evident that the Aigialosaurs show great affinity to the 



^ This classification does not correspond with the one given by Gorjanovic- 

 Kramberger. 



