346 PROFESSOR OWEN ON THE GENUS DINORNIS. 



groove is 1^ line: near the commencement, the hinder wall of the groove shows an 

 oblong vacuity. 



The fore part of the basisphenoid platform is impressed by a pair of curved, shallow 

 fossae, concave forward. The substance of the basisphenoid is pneumatico-cellular : on 

 removing the thin compact crust of the lower surface, as in PI. LIV. fig. 1, the wall 

 of the hemispheroid pituitary fossa or ' sella ' (PI. LVI. fig. 1 , d) is shown, demonstrating 

 the boundary between the basi- and pre-sphenoids. 



The lower surface of the basisphenoid platform is gently concave lengthwise and 

 transversely between the produced anterior angles ; but, across the mid part, it is con- 

 cave medially and convex laterally. The lateral margins, forming the lower part or 

 floor of the medial extension of the tympanic cavity, are sharp and jagged, concave 

 lengthwise, between the mammillar (i') and pterapophysial (s') productions, where they 

 are impressed by the Eustachian channels. The tract of bone (r) from the outside of 

 the mammillar protuberance to the lower end of the paroccipital ridge forms the back 

 part of the beginning of the Eustachian groove (e) and the fore part of the carotid 

 fossa (c). 



The mastoid sends off three processes in many birds, the ' mastoid ' process proper ( 8 ), 

 the 'post-tympanic ' (s"), and the pretympanic (s'). The post-tympanic is the longest 

 in Dinornis, and the shortest in Aptornis^ and Didus. The pretympanic process is very 

 long in Notornis and Porphyria^. 



Internal to the pretympanic process and between it and the post-tympanic, the 

 mastoid forms, by a sharp ridge, the outer and front boundary of the anterior fossa for 

 the condyle of the tympanic bone (28). The mastoid process (s) projects from above 

 the base of the post-tympanic {»"), and is tuberous and rough. 



The paroccipital (ib. 4) with the post-tympanic (s") forms a smooth arch of bone over- 

 hanging the membrana tympani : the anterior surface of the paroccipital, forming the 

 back part of this arch, is divided by the narrow ridge for the attachment of the ear- 

 drum into the ectotympanic and entotympanic surfaces. 



The tympanic cavity (ib. fig. 1, t, u) is of a triangular form, bounded externally 

 by this ridge, the post-tympanic process, and the tympanic plate of the mastoid, 

 posteriorly by the paroccipitals (ib. 4), internally by the basisphenoid (ib. 5), and an- 

 teriorly by the alisphenoid (ib. e). The cavity presents a most irregular surface. On 

 the outermost part of the roof, immediately within the ear-drum, is a large oval 

 pneumatic foramen, immediately mesiad of which is part of the single, deep, oblong, 

 smooth, articular cavity (u) for the tympanic bone, 10 lines in length and 5^ lines in 

 breadth at its anterior and widest part ; whence it extends inward and backward from 

 the mastoid to the paroccipital. Anterior to and mesiad of the articular cavity is 

 a second large oval pneumatic vacuity (ib. fig. 1, u) leading to a vertical fossa in 

 the cranial wall, homologous with that wider and more conspicuous ' pretympanic ' 

 ' Zool. Trans, vol. iii. pi. 52, s". ^ Ib. pi. 56. figs. 1 & 7. s". 



