28 



BULLETIN 89, UNITED STATES NATIONAL MUSEUM. 



openings are oval in outline and are in a line nearly parallel with the top of the 

 skull." 



The great vertical depth of the median part of the lower jaw is especially note- 

 worthy. A measurement taken below the orbit shows it to be over one-half the 

 whole depth of the head, an unusual proportion in dinosaurian anatomy. 



From a superior view the wedge-shaped form of the skull is still apparent. 

 The only risible openings are the small subcircular supratemporal fossae. There 

 are no supraorbital vacuities as found m Oamptosaurus and Iguanodon, this region 

 being entirely roofed over by the supraorbital and postfrontal bones. This view 

 is well shown in plate 6. 



Viewed from the back, as shown in plate 9, figure 2, the skull and mandible 

 present a nearly quadrate outline. 



In its general proportions and outlines the skull of Stegosaurus is found to resemble 

 that of Oamptosaurus more nearly than that of any other American dinosaur, though 



it should be borne in mind 



■/tec 



Fig. 4. — Posterior view of the skull of Stegosaurus stenops? Marsh. 

 Cat. no. 2274, U.S.N.M. i Nat. size, bo, Basioccipital; Ex.o, exoccipital; 

 p, parietal; p.oc, paraoccipital; so, sUPRAOcciPiTAr.; Sq, squamosal. 



that the shape and arrange- 

 ment of their constituent ele- 

 ments differ considerably. 



Basioccipital (bo.). — The 

 basioccipital is terminated 

 posteriorly by a moderately 

 broad, rounded occipital con- 

 dyle. The smooth articular 

 surface is continued forward 

 on the underside (pis. 5 and 

 7, oc). The condyle in No. 

 4934 is considerably narrower 

 transversely than in skulls 

 Nos. 2274 and 6645, as may be seen by comparing text figures 4 and 5 with figure 

 2, plate 9. The occipital condyle is inclined ventrally to the longer axis of the 

 skull, so that when articulated with the neck the longer axis of the skull would 

 form an obtuse angle with the axis of the anterior cervical vertebrae, as it does in 

 Oamptosaurus, Tracho&on, Triceratops, and Diplodocus. 



The basioccipital articulates, as shown by the detached element in specimen 

 No. 4935, with the basisphenoid by a deep vertical suture, without a median tongue- 

 like extension, as found in Oamptosaurus. In front of the condyle the inferior 

 surface is deeply concave longitudinally, and convexly rounded transversely, and 

 without median pit, as found ia some predentate dinosaurs. On the anterior 

 median part of this surface is a notch between the blunt basioccipital processes. 

 These abut against simdar processes on the posterior end of the basisphenoid (pi. 7) . 

 The median line of the superior surface is shallowly concave transversely and forms 

 the floor of the foramen magnum. On either side are the beveled, sutural surfaces 

 for articulation with the exoccipitals. 



The exoccipitals, as in Oamptosaurus, participate in the formation of the 

 occipital condyle. (See fig. 4.) 



Exoccipital (ex. o.) and Paraoccipital (p. oc). — The exoccipitals are the largest 

 bones of the occipital segment. They form the lateral boundaries of the foramen 



