362 PROFESSOR OWEN ON THE GENUS DINORNIS, 



which sends downwards two processes, both having a ridge on their outer surface : the 

 hinder or ' tympanic ' process (s", figs. 1 & 3) is the smallest, and bends in towards the 

 tympanic tuberosity of the basisphenoid {y, fig. 3); the anterior or proper mastoid process 

 (fig. 1,8') is an inch in length and a third of an inch in breadth, compressed from before 

 backwards, grooved along its thicker inner border, and slightly bent forwards at its 

 pointed extremity : a small part of the anterior margin coalesces with the long recurved 

 postfrontal (12) circumscribing the temporal fossa externally, as in the Crocodiles. 

 This coalescence does not take place in either Otis or Didus. The mastoid in Otis sends 

 down a short tympanic process (fig. 9,8"), as well as a long and rather slender mastoid 

 process (s) : and the base of the mastoid has two articular cavities for the upper condyles 

 of the tympanic bone. In Didus the outer side of the mastoid is convex, smooth, but 

 with a slight oblique ridge : it overhangs the tympanic cavity, bending inwards, and 

 sends a short compressed pointed mastoid process in front of the anterior articular 

 cavity for the anterior superior condyle of the tympanic. The inner surface of the base 

 and proper process of the mastoid affords three distinct articular surfaces for the tym- 

 panic bone : two of these are concave, are separated by a pneumatic foramen, and 

 receive the proximal condyles of the tympanic ; the third (y", fig. 3), on the inner border 

 of the mastoid process, is flat, and articulates with a well-defined elliptic surface 

 (/, fig. 9. PI. LIII.) on the outer side of the tympanic. 



The tympanic cavity presents a large smooth transversely oblong depression (d, fig. 3) 

 at its back part, bounded behind by the paroccipital ridge (4') and the anterior external 

 tubercle of that ridge, and bounded anteriorly by the tympanic process (8") of the mas- 

 toid, which continues the curve of the anterior border of the depression which gave 

 attachment to the tympanic membrane ; on the inner side of this semicircular border 

 are the orifices of the ' meatus internus ' (m, fig. 3), those of the eustachian canals, and 

 of pneumatic foramina communicating with the cellular structure of the walls of the 

 tympanic cavity. 



The presphenoid (9) is a deep compressed plate, with a rounded thickened border 

 below, expanding as it advances as far as the prefrontal plate, where its anterior end 

 has been broken off. The hinder parts of the palatines (20) where they coalesce with 

 the vomer abut against the sides of the presphenoid. 



The orbito-sphenoids (10) are indicated by the large optic foramina (0 p), behind which 

 are three smaller separate holes for the nerves and vessels of the orbit : external to these 

 is a larger foramen, terminating a groove continued from the foramen ovale, and repre- 

 senting, therefore, probably the ' foramen rotundum ' for the second or suborbital divi- 

 sion of the trigeminal nerve : between this and the eustachian outlets there is a smtiller 

 foramen. 



The orbital plates of the frontal (11', fig. 3) are more remarkable for their transverse 

 than their antero-posterior extent, and form at their meeting with the cranial plates 

 (11, figs. 1 & 2) a broad overhanging supraorbital ridge, notched anteriorly behind a 



