OSTEOLOGY OF THE AEMOEED DINOSAUKIA. 33 



gosaurus, as plainly shown by specimen No. 6645. On the lower lateral surface 

 this bone is pierced by a small foramen (VII, fig. 10) for the exit of the seventh or 

 facial nerve. More posteriorly the prootic forms the anterior border of the internal 

 auditory meatus (VIII, fig. 10). 



Lachrymal 1 (I.). — The lachrymal as seen in specimen No. 4934 (see I, pi. 5) 

 is a small, irregularly shaped bone lying wedged in between the maxilla, nasal, 

 jugal, and prefrontal. It is overlapped on the upper posterior border by the 

 presupraorbital. 



Supraorbitals (p.so and po.so). — The supraorbital in Stegosaurus is com- 

 posed of two separate pieces, which as a matter of convenience, in referring to 

 them, I shall designate as the pre- and post-supraorbital bones (see p. so and po. 

 so, pi. 6). In Scelidosaurus as described by Owen in his Fossil Reptilia of the 

 Liassic (p. 10), the supraorbital is present but consists of a single bone. 



Marsh, 2 as shown by his figures of the skull, based upon specimen No. 4934, 

 has considered the supraorbital as consisting of one element. 3 In this, however, 

 he is in error, as clearly shown by two partly disarticulated skulls, Nos. 6645 and 

 4935. In fact, now that it is known that there are two bones in this region, the 

 sutures appear quite distinct in No. 4934, and the outlines of the elements can be 

 clearly traced, as indicated in plate 6. These elements make a complete roof 

 over the orbit, there being no supraorbital fossa as found in Camptosaurus and Igu- 

 anodon bernissartensis. The articulated supraorbital bones are well shown in the 

 skull of S. stenops No. 4934. From a lateral view, see fig. 3, the postsupraorbital is 

 a small triangular bone which overlaps the antero-lateral surface of the postorbital. 

 It forms the rounded postero-dorsal boundary of the orbit, and extends forward 

 above the orbit nearly to its center where it unites by a straight transverse suture 

 with the presupraorbital. The presupraorbital is a comparatively long, narrow, 

 curved bone which completes the upper boundary of the orbit, and unites in front 

 with the prefrontal and lachrymal. As shown in plate 6, these two elements 

 form the external boundary of the upper half of the orbit. 



Nasals (n.). — The nasals are long bones and form the greater portion of the 

 upper surface of the snout. Posteriorly they articulate with the frontals by over- 

 lapping finger-like processes, while in turn they are overlapped on this end by the 

 prefrontals. (See pi. 6.) 



Laterally, the nasals turn abruptly down to unite with the maxillae and pre- 

 maxillae, and are only slightly in contact with the lachrymals on the infero- 

 posterior angles. The pointed anterior ends are overlapped by the backwardly 

 directed processes of the premaxillae. The upper surfaces of these bones are 

 flattened throughout the greater part of their length, although transversely 

 rounded above the narial orifice. The nasal in specimen No. 4934 measures 248 

 mm. in length. 



Premaxillae (pm.). — In Stegosaurus, as in all known predentate dinosaurs 

 excepting Hypsilophodon foxii, the premaxillary bones are edentulous. In skull 



i Adlaehrymal of Gaupp. Beitrage zur Kenntnis des Unterkiefers der Wirbelthiere. Anat. Anz., vol. 39, 1911. 



» Amer. Journ. Sci., ser. 3, vol. 34, 1887, pi. 6, figs. 1, 2, 3. 



3 Catalogue of Fossil Reptilia and Amphibia, pt. 1, 1888, p. 176. Lydekker ina footnote comments on the skull of Stegosaurus 

 as follows: " This hone forming the upper border of the orbit and connecting the pre- and post-frontals appears to be an element 

 usually not represented as a distinct bone." 



