614 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 49. 



Wealden and the Lance, these teeth have disappeared in the some- 

 what more speciahzed Thescelosaurus. 



The presence in Thescelosaurus of single-headed posterior thoracic 

 ribs and four phalanges on digit III of the fore foot are structural 

 features sufficient to separate it from all known dinosaurian genera 

 with the exception of Hypsiloj^Jiodon, and I would therefore assign 

 Thescelosaurus to the family Hypsilophontidae, 



The combination of characters now considered diagnostic of this 

 family are: 



Premaxillaries with teeth. Digit III of the hand with four phalanges. Femur 

 either longer or shorter than tibia. Posterior thoracic ribs single-headed. Genera. — 

 Hypsilophodon Huxley from the Wealden of the Isle of Wight and Thescelosaurus Gil- 

 more from the Lance formation of Wyoming. 



For obvious reasons the genera Nanosaurus, Laosaurus, and Dryo- 

 saurus should be removed to the family Laosauridae. 



The genus Tliescelosaurus may be distinguished from Hypsilopho- 

 don by the greater length of the femur over the tibia and by the 

 position of the fourth trochanter below the middle of the femur, 

 whereas in Hypsilophodon this trochanter is entirely upon the proximal 

 half of that bone. 



The chief similarities and differences in the skeletal structure of 

 the two genera constituting the family Hypsilophontidae are con- 

 trasted in the parallel columns below: 



Hypsilophodon . 



Skull: 



Premaxillaries with teeth. 



Maxillaries with a single row cf teeth. 



Anterior vertebrae platycoelian. 



Sacrum of five vertebrae coossified. 



Posterior thoracic ribs single headed. 



Sternum ossified. 



Scapula elongated with moderately 

 expanded blade. 



Manus with five digits, metacarpals 

 moderately lengthened. 



Third digit with four phalanges. 



Femur longer than tibia. 



Fourth trochanter on proximal half of 

 femur. 



Pes with five digits, metatarsals elon- 

 gated. Met. V vestigal. 



Rounded rodlike pubis with long 

 slender postpiibis. 



Ischium with obturator process near 

 center of shaft. 



First 14 caudals with transverse proc- 

 esses. 



Pubic foramen closed. 



Sacral ribs uniting with sacrum inter- 

 vertebrally. 



Thescelosaurus. 



Unknown. 



Unknown. 



Same. 



Same. 



Same. 



Same. 



Scapula shortened with expanded 

 blade. 



Manus with five digits, metacarpals 

 shortened. 



Same. 



Femur shorter than tibia. 



Fourth trochanter extending below 

 middle of femur. 



Pes with five digits, metatarsals short- 

 ened. Met. V vestigal. 



Same. 



Ischium with obturator process nearer 

 proximal end. 



First 10 caudals with transverse proc- 

 esses. 



Public foramen not closed. 



Sacral ribs uniting with sacrum in part 

 intervertebrally and part direct. 



