512 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [June 25, 



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 unmis- 

 takable. 



The well-developed brain, expanding transversely in its poste- 

 riorly placed box (PI. XVI. figs. 1-4, 3, 7, 11), making the base of a 

 long cranial cone gradually tapering forward, the capacious lateral 

 orbits (ib. figs. 1, 2, 4, 0, o), and the single hemispheroid condyle (ib. 

 fig. 3, 1) are avian : the large and long, freely articulated, dependent 

 tympanic bone (ib. figs. 1, 2, 3, 28), the slender, straight and styliform 

 zygomatic bar (ib. figs. 1 & 2, 26) received behind into the articular 

 cup of the tympanic (ib. figs. 1 & 3, h) — 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 suspi- 

 cion of the significant and well-marked modification of the mandibles 

 which has suggested the generic name I have proposed for this ex- 

 tinct Eocene bird. 



The occipital region (PI. XVI. fig. 3) is broader than it is high ; 

 the occipital foramen (ib. m) partakes of the same proportion; the 

 transverse diameter also exceeds in the condyle (ib. 1), 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. 3) 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 protu- 

 berance 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. 2), passing obliquely 

 from near the side of the foramen magnum to the paroccipital wall 

 (fig. 1, 4) 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, 

 x, x) 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| lines (0-022 m.), to the upper border of the 

 occipital foramen 6| lines (0-015 m.); the extreme breadth (trans- 

 verse diameter) of the occiput is 1 inch 3 bines (0-032 m.) ; the 

 transverse diameter of the occipital foramen is 4 bines (0*008 m.). 



The portion of the atlas (PI. XVI. fig. 3, c) 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, 7) slightly rises as it advances 

 from the superoccipital ridge to the interval between the postor- 

 bitals, 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 



