386 MB. w. p. PTCEAET ON THE [Mar. 21, 



bone from the dorsal region of the rim of the optic foramen 

 forwards to the interorbital plate. 



^ The orhitosphenoid in the Diomedeidse is completely ossified ; 

 the roof of the orbit is more or less perfectly protected by a 

 supra-orbital ledge, such as occurs in the Penguins. 



The optic foramina in Diomedeidse, Ossifraga, Fulmarus, 

 PrioceUa, Prion, Da'ption, Thalassoeca, and some species of Puffinus 

 and CEstrelata, are divided by a median septum one from another ; in 

 the rest the septum is absent and the two apertures are confluent. 



The ethmoidal region. — The mesethmoid is indistinguishably 

 fused below with the parasphenoidal rostrum, from Avhich it rises 

 as a thin vertical plate of bone, in the median line. Its dorsal 

 border is expanded so as to underlie the nasal and frontal bones, 

 extending outwards on either side to the level of the free edge of 

 the supraorbital groove. The anterior border of the mesethmoid 

 is of considerable width ; postero-dorsally it extends backwards to 

 play the part of a crista-galli dividing the olfactory fossce into right 

 and left lateral chambers ; its postero-ventral border is merged 

 with the interorbital septum when present. The aliethmoid is 

 only the ectoethmoidal ossification and forms the antorbital plate. 

 This, in the Procellariidte, is generally of very considerable size. 

 In Bidweria it takes the form of an almost vertical plate of bone, 

 projecting nearly at right angles from the posterior border of the 

 mesethmoid, and running outwards to the lachrymal. Its dorsal 

 border becomes continuous with the free edge of the expanded 

 mesethmoid. Its postero-dorsal angle is more or less hollowed 

 out and trends downwards to join the median horizontal bar of 

 bone representing the interorbital septum. This antorbital plate 

 serves to enclose two spacious olfactory chambers divided one from 

 another by the mesethmoid. Anteriorly they are in direct 

 communication with the lachrymo-nasal fossa, posteriorly with the 

 brain-cavity. Procellaria, Oceanites, and Cymodroma more or less 

 resemble Bulweria in this respect. In Fulmarus, Thalassoeca, 

 CEstrelata, Daption, and Prion the form of the antorbital plate 

 resembles that just described. In these genera, however, the 

 outer border is fused with the lachrymal. In all the genera 

 so far enumerated the dorsal border of the lachymal is pierced by 

 two foramina. Of these, one lies immediately under the free 

 edge of the frontal, and the other between the lachrymal and the 

 aliethmoidal wall. In Priocdla these two foramina are merged 

 into one, forming a deep emargination between the dorsal wall of 

 the antorbital plate and the frontal ; externally this plate and the 

 lachrymal are fused as in Fulmarus, &c. The outer of these two 

 foramina — the lachrymal — in Priofinus, Majaqneus, and Pujflnus is 

 of great size ; in all except a few species of Ptiffinus the antorbital 

 plate remains distinct from the lachrymal. 



In the Diomedeidse the autorbital plates are represented by a 

 pair of narrow lateral wings, which never extend dorsally to meet 

 the frontal. In Phcehetria they extend laterally so as to pass 

 behind, and project slightly beyond, the level of the lachrymal. 



