OCR Text |
Show 1899.] OSTEOLOGY O F T H E TUBINABES. 389 division of the hemispheres dorsally. The cerebral fossa is of much greater relative size in the Diomedeidas. The olfactory fossce are paired tubular cavities lying immediately in front of the cerebral fossas, and leading out into the olfactory chamber by a wide aperture. The Premaxilla. The premaxilla-and, as will be shown presently, the whole facial skeleton-closely resembles that of the Ciconiiformes. In the Tubinares it is in all cases more or less produced forwards and strongly hooked at the tip. In breadth it varies. In the Procellariidas, amongst the smaller forms, e. g. Oceanites, what is probably the more primitive form of this region of the skull obtains, in that we can distinguish the three radiating prongs by which the premaxilla is bound to the rest of the jaw, viz., the median, paired, nasal processes and the lateral maxillary processes. In Oceanites, Cymodroma, &c. these are long and narroAv and wide apart. Thus we get a long, median palatal vacuity, and elongated, paired, but horizontal and pervious nares. The nasal processes fuse proximately with the nasal bones and are never more than indistinctly to be made out in this region. The outer border of the maxillary processes in the larger Procellariidas, e. g , Fulmarus, Priofinus-aided by the maxilla-take the form of vertically flattened plates, which in Prion become laterally expanded so as to make the beak boat-shaped-as in Balceniceps and Can-croma amongst the Ciconiiformes. The great development of these vertical plates causes the narial apertures to look upwards, rather than outwards as is usual. Moreover, it gives the jaw the appearance of great solidity, which attains its climax in the Diomedeidas. In all belonging to this subfamily-save the genus Puffinus- as already indicated, there is a large vacuity immediately distad of the maxillo-palatine processes and extending forwards to the tip of the jaw. In the genus just referred to as the exception to this rule, the vacuity is represented by a wide chink, not extending forwards further than the middle of the jaw, where the edges of the crevice meet to form a bony roof to this region of the mouth. There is an approach to this condition in Priofinus and Majaqueus. The palatal surface of the maxillary processes attains its maximum breadth in Prion and Pelecanoides. In the Diomedeidas this premaxillary vacuity is reduced to a long narrow chink extending about as far as the middle of the jaw, when, as in Puffinus, the edges meet to form a bony palatal roof. The Maxillo-jugal Arch. As in the Ciconiiformes, the maxilla, in the adult, is indistinguish-ably fused with the premaxilla. The maxillo-palatine processes, in Oceanites, Cymodroma, and Procellaria, are represented by delicate horizontal, more or less fenestrated, leaf-shaped expansions approaching one another in the middle line. In the rest of the PROC ZOOL. Soc-1899, No. XXVI. 26 |