PROTOSTEGIDJE. 



205 



Points to be especially noted in this skull are its long and narrow form, the long preorbital 

 region, the hookt beak, the posterior position of the orbits, and the upwardly directed nasal 

 opening. Contrary to what is usually seen in turtles, the orbits are placed in the middle of the 



pmx 



Cttlfl 



FIG. 263. Archelon ischyros. Skull. Xj. 



ang, angular; art, articular ; den, dentary; fr, frontal; ju, jugal ; mx, maiilla ; nar, 'nares; pr/,"pref rental; pal, 

 palatine; pmx, premaiilla; pa, parietal; pof, postfrontal; qu, quadrate; quj, quadratojugal; sq, squamosal; 

 st, stapedial rod ; jar, supraangular; vom, vomer. 



length of the head. This results from the great development of the preorbital region. The 

 horizontal diameter of the orbit is 150 mm. 



Both the maxillae and the premaxillae are greatly prolonged. The distance from the tip 

 of the beak to the anterior rim of the orbit is 240 mm.; from the orbit to the suture between the 



pmx 



FIGS. 264 AND 265. Archelon ischyros. Skull. 



264. Palatal surface. X J'Q. bsp, basisphenoid; cho, cho- 



anse; ju, jugal; mx, maxilla; oc, occipital condyle; 

 pal, palatine; pmx, premaxilla; ft, pterygoid; qu, 

 quadrate; quj, quadratojugal ; sq, squamosal; vom, 

 vomer; vp, vomero-palatine region. 



265. Side view of front. Xj. mx, maxilla; nar, nares; 



o, orbit; pal, palatine; pmx, premaxilla; prf, pre- 

 frontal; vom, vomer. 



maxilla and the premaxilla is about 150 mm. The tip of the premaxillae is bent down at nearly 

 a right angle with the axis of the skull, forming a great hawk-like beak. The nasal opening 

 looks wholly upward. Its length is 120 mm.; its width, 70 mm. 



