tion the jugular foramen and possibly the foramen rotundum.

The exit for the facial nerve, generally small in ceratopsians, appears unusually so in Pachy- rIIinosaurus. The foramen with a diameter of 3 mm occurs a short distance in front of the lower corner of the 'fenestra ovalis' in NMC 9485. Except for the problematical route of the chorda tympani, no indication of the paths of this nerve's branches appear on the lateral walls of the braincase. The largest of the lateral cranial foramina lies 9 mm anterior to the facialis and, as in other ceratopsians, is believed to have transmitted the maxillary and mandibular rami of the trigeminal nerve. In large ceratopsids the ophthalmic ramus turned forward within the braincase wall to emerge some distance antero- dorsal to the main trigeminal foramen. This opening is not definable in NMC 9485, but a number of small openings occur in the area where it should be expected.

Small foramina, presumably for nerves VI and III, exit laterally about as figured in Triceratops (Hay 1909, Pl. I) below the thick 'antotic but- tress'. The fourth nerve is believed to have emerged higher up on the side of this buttress. The optic foramen is only a little smaller than the trigeminal. It lies anterior to and in line with exits VI and III.

The only other cranial foramen that can be identified in the specimen transmitted the in- ternal carotid artery. This landmark enters the basicranium lateral to the sharp ventral edge of the basisphenoid. The area nearby is crushed on the right side (the left side is not exposed), and it is impossible to state whether there is a sulcus leading into the foramen. The opening is, how- ever, relatively and absolutely smaller than in the Chasmosaurus skull or in Triceratops. It is also situated much lower down on the side of the cranium.

Mandible — The Scabby Butte collection con- tains an anterior end of a dentary bone and an associated predentary that were found with parts of a skull (NMC 10645) about a quarter of a mile ( 400 m) from Site 2. This was an isolated occurrence, and there is no reason to suppose that the fragments do not belong to one indivi- dual. The dentary fragment containing the bottoms of the first twelve alveolar grooves agrees with a more complete dentary from Site 2

CAN. J. EARTH SCI. VOL, 12, 1975

Fio. 7. I'achyrhinosaurus canadensis Sternberg. NMC

1064S, right dentary, lateral aspect. Scale: 10 cm.

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{NMC 10643). The Site 2 specimen (Fig. 7) was found close to the skull NMC 9485 and seems to correspond with it in length; it may well have belonged to the same individual. Its dorsal edge has been destroyed, and the dental magazine is lost, and, as preserved, the bone appears relatively more slender than the dentary in the paratype of P. canadensis (NMC 8866). But both jaws are relatively deeper than any

Pachyrhinosaurus skull, The other two passages unite into a common tunnel within the cranial wall. This passage then communicates antero- proximally with the long, narrow slit represent- ing the persistent metotic fissure. Endocranial casts and probing in this specimen suggest that I tubular structures entering the metotic fissure

from different places within the endocranium emerged partly through the more anterior of the two so-called hypoglossal foramina (the jugular foramen of Hay) and partly through the opening 5 that is usually termed the fenestra ovalis in

ceratopsians. One interpretation of' this arrange- 4

ment would have all branches of the hypoglossal nerve exiting through a single posterior foramen, with nerves IX-XI emerging from the jugular foramen nearby. However, it seems more likely O that the three serially aligned foramina in the

CIIasmosaurus end ocr anium carried separate branches of the twelfth nerve, and that two anterior branches, joining with the tenth and eleventh nerves from the metotic fissure, exited o g with these through a common passage behind the ~ o plane of the transverse occipital plate. The ninth

cranial nerve, on the other hand, {possibly accompanied by the tenth), remained in the

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bony metotic fissure, eventually emerging via the large 'fenestra ovalis' of authors. (I have o o shown (Langston 1960) that the glossopharyn- geal nerve probably passes close to the fenestra ovahs in a hadrosaur.) Possibly the perilym- phatic duct was also associated with the large

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lateral opening as well. This opening occurs in 4 pachyrhinosaurus in about the same position as

the so-called fenestra ovalis of Triceratops (Hay 1909, Pl. I). It is vertically oval with diameters

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of 29 and 9 mm. The corresponding opening in the Chasmosaurus cranium is funnel-shaped, 4 being widest laterally. Not only does it communi- 'o cate directly with the metotic fissure, but the

cavum capsularis opens into it via the true fenestra ovalis deep within the braincase wall; the fenestra ovalis is not visible in lateral aspect, but what I suppose to be a stapedial groove passes out of the large opening and extends a short distance laterally across the anteroventral surface of the paroccipital process in NMC 9485, and from its anteroventral corner a sulcus pos- sibly marking the route of the stapedial artery may be followed along the basioccipital-basis- phenoid contact. Hence the large ceratopsid fenestra ovalis of authors is, distally, much more

r c coins cine inure r r ~mn A Cw

saurus; although more massive, it seems to have about the same proportions as the surangular in the holotype of Centrosaurus lottgirostris Sternberg {NMC 8795). The small excavation that received part of the lateral quadrate con- dyle is distorted but would apparently have accommodated the quadrate described above. An interesting divergence from the pattern seen in Styracosaurus and Centrosaurus occurs at the posteromedian edge of the ascending ramus. In those genera, this edge is thin and sharp and curves broadly mesad to pass underneath the anterolateral corner of the articular bone. The corresponding edge in NMC 10629 is thick and is emarginated by a wide notch over which the posterior adductor muscle probably passed on its way to the adductor fossa {see Ostrom 1964). This edge is broken oA' in the paratype of P. canadensis.

The predentary bone of Pachyrhinosaurus has

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