170 University of Kansas Geological Survey. 



having been thoroughly studied, and all compared with the po- 

 sitions of the different bones in the perfectly preserved and un- 

 distorted skull of Mosasaurus horridus from the Fort Pierre. 

 That all errors have been avoided in the restorations I cannot 

 hope for ; I am confident, however, that they have been re- 

 duced to a minimum. The limb bones are invariably flattened, 

 and rarely agree exactly. If one takes into account these differ- 

 ences, scarcely ever will any two specimens be found alike ; if 

 one ignores them four-fifths of all the described species must be 

 abandoned. 



MOSASAURIA. 



Pythonomorpha Cope, Proc. Bost. Soc. Nat. Hist., 1869, p. 253. 



Large marine reptiles, varying in length between five and 

 forty feet. Limbs wholly natatory, webbed to the extremity of 

 the digits, and without claws, the phalanges often numerous, 

 the arm and leg bones short and broad, and incapable of rota- 

 tion upon each other, or so to a limited extent only. Pectoral 

 girdle composed of scapula, coracoid, and (in some cases) an in- 

 terclavicle, the clavicle never present. Sternum, when present, 

 calcified. No sacrum ; the anteriorly directed, rod-like ilium 

 feebly or not at all united to the vertebral column. Pubes and 

 ischii united in a ventral symphysis. Vertebrae procoelous, 

 from 115 to 150 ( ?) in number. Cervical vertebrae seven in num- 

 ber, the first five or six with an articulated hypapophysis. Tho- 

 racic ribs not more than fifteen in number, single headed, 

 attached to a stout diapophysis springing from the centrum. 

 All the precaudal vertebrae, except the atlas, costiferous. Zyg- 

 apophyses stout and strong anteriorly, becoming obsolete at 

 the beginning of the tail, Tail elongate, as long as the pre- 

 caudal series of vertebrae, more or less compressed, with 

 elongated chevrons, save at the basal part, and with diapophy- 

 ses anteriorly. Premaxillaries united with each other and with 

 the coossified nasals. Frontal and parietal bones unpaired. A 

 large pineal foramen present. Parietal with decurved, wing- 

 like processes for union with petrosal and supraoccipital. 

 Jugal arch complete. Temporal arch composed of squamosal, 



