Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/590

512 careful and skilful hands of Mr. Davies, Senior Attendant in the "Geological Department," brought to light the tooth-like processes of the alveolar borders of both upper and lower jaws, to which the uniqueness of this Eocene fossil is due; but the distinctive cranial characters of the warm-blooded feathered vertebrate are unmistakable.

The well-developed brain, expanding transversely in its posteriorly placed box (Pl. XVI. figs. 1–4, ), making the base of a long cranial cone gradually tapering forward, the capacious lateral orbits (ib. figs. 1, 2, 4, ), and the single hemispheroid condyle (ib. fig. 3, ) are avian: the large and long, freely articulated, dependent tympanic bone (ib. figs. 1, 2, 3, ), the slender, straight and styliform zygomatic bar (ib. figs. 1 & 2, ) received behind into the articular cup of the tympanic (ib. figs. 1 & 3, )—all the modifications, in short, that relate to the free and characteristic movements of the beak—are likewise here present.

Nothing in the fossil, at first apparent, could have led to a suspicion of the significant and well-marked modification of the mandibles which has suggested the generic name I have proposed for this extinct Eocene bird.

The occipital region (Pl. XVI. fig. 3) is broader than it is high; the occipital foramen (ib. ) partakes of the same proportion; the transverse diameter also exceeds in the condyle (ib. ), of which hemisphere the upper part is truncate. The upper border of the foramen, through the posterior swelling of the cerebellum, slightly overhangs the condyle. The cerebellar protuberance (ib. ) seems to have had a vertical median ridge, as it shows the broken or worn base of such a prominence. On each side of the cerebellar protuberance the occipital surface is smooth and moderately concave across; it is, in a less degree, convex vertically, until it bends in below to the upper border of the occipital foramen. The beginning of the subvertical exoccipital prominence (ib. ), passing obliquely from near the side of the foramen magnum to the paroccipital wall (fig. 1, ) of the tympanic cavity, is preserved; but the paroccipital itself is broken away. The upper transverse occipital ridge, low and linear, arches outward from the top of the vertical ridge (fig. 3, ) on each side down to the broken base of the paroccipital.

The depth (vertical diameter) of the occiput to the lower border of the condyle is 10$1⁄2$ lines (0⋅022 m.), to the upper border of the occipital foramen 6$1⁄2$ lines (0⋅015 m.); the extreme breadth (transverse diameter) of the occiput is 1 inch 3 lines (0⋅032 m.); the transverse diameter of the occipital foramen is 4 lines (0⋅008 m.).

The portion of the atlas (Pl. XVI. fig. 3, ) preserved, as dislocated from the condyle below the foramen magnum, closely conforms to the avian type of that vertebra.

The parietal region (ib. figs. 1, 2, 3, 4, ) slightly rises as it advances from the superoccipital ridge to the interval between the postorbitals, when the frontal surface passes forward with a slight convex curve to between the large orbits, and gradually sinks as it goes straight to the transverse fronto-nasal suture (ib. fig. 4, ). The