386 ME. w. p. pycEAFT 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, 



Priocella, Prion, Daj^iiion, Thalassoeca, and some species of P^tffinus 



and (Estrdata, 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 mcsethmoid is indistinguishably 



fused below with the parasphenoidal rostrum, from which 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 latt-ral 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 Proeellariidae, is generally of very considerable size. 



In Bidiveria 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 Cijniodroma more or less 



resemble Bidweria in this respect. In Fuhnarus, Jlialassceca, 



(Estrelata, 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 Priocella 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 Fidmariis, &c. The outer of these two 



foramina — the lachrymal — in Priofinus, Majaqueus, and Puffinus is 



of great size ; in all except a few species of Puffinus 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 Phoebetria they extend laterally so as to pass 

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



