388 PEOr. T. JEFFEEY PAEKEli ON THE CEANIAL OSTEOLOGY, 



the olfactory chambers. There the Dinomithidce show, to a less degree, the state of 

 things which reaches its maximum among birds in A'pteryx. 



As already remarked, the upper margin of the orbit, formed by the supraorbital 

 ledge, may be either evenly curved, sinuous, or right-angled. The roof of the cavity 

 formed by the projecting ledge is almost horizontal from within outAvards ; its mesial 

 wall slopes from the supraorbital ledge downwards and inwards to the optic and 

 lacerate foramina, the slope being far steeper in the narrow-skulled genera than in the 

 wide-skulled Binornis. 



Both roof and mesial wall of the orbit are pierced with vascular foramina, but the 

 first feature of importance met with in this region of the skull is the optic foramen 

 {op.for.). This lies in the postero- ventral region of the orbit, slightly in advance 

 of the base of the rostrum. In Emeus it is a nearly circular aperture directed from 

 the cranial cavity outwards, forwards, and upwards, and its lower margin is separated 

 from that of its fellow of the opposite side by a distance of about 9 or 10 mm. In 

 Binornis it is proportionally smaller and oval in outline ; its lower margin is formed 

 by a thin obliquely horizontal shelf of the presphenoid, represented in the other genera 

 by a mere ridge, and the distance between the two foramina is about 30 mm. 



The greater part of the inner wall of the orbit is furnished by the orbital plate of 

 the frontal (fig. 18), which, in the young skull, articulates below with the inferior 

 aliethmoid {inf.al.eth.) and the orbitosphenoid (or.sjjh.) ; the suture passes about 

 2 mm. above the optic foramen, which is therefore bounded above by the orbito- 

 sphenoid, below by the presphenoid. 



Bounded above by the shelf or ridge forming the lower margin of the optic foramen, 

 and below by the rostrum, is a depression, the liresjjhenoid fossa (fig. 18, pr.sph.), 

 very obvious in Binornis, Pachyornis, Anomalopteryx, rather obscure in Mesoj)teryx 

 and Emeus. The vertical plate of bone separating the fossae of the right and left 

 sides is very tliin, and is the only indication of an interorbital septum, thus showing 

 an interesting approximation to the typical avian structure of the presphenoid. 



Immediately posterior to the optic foramen is the aperture called by Owen the 

 prelacerate foramen or foramen rotundum, but more conveniently named the lacerate 

 fossa, as it is not a simple foramen, but a pit including three perfectly distinct foramina. 

 One of these (Plate LXII. fig. 76, v'^), placed postero-dorsally, is the opening of a 

 tunnel-like excavation in the alisphenoid, and transmits the orbitonasal nerve. The 

 second (iii), below and in front of the first, is bounded behind by the alisphenoid and 

 in front by the basisphenoid, and transmits the oculomotor nerve. The third (vi&«), 

 situated below and in front of the second, perforates the alisphenoid and probably 

 transmits the internal ophthalmic artery ; it has in its hinder margin the aperture of a 

 canal through which the sixth nerve enters the orbit. The fourth nerve (iv) enters 

 through a very small foramen placed just above that for the third nerve, and quite 

 outside the lacerate fossa. 



