PROFESSOR OWEN ON THE GENUS DINORNIS, 347 



fossa which characterizes the cranium in Sula and other Pelecanidce. From the back 

 part of this extends the groove for the tympanic vein leading to the postlacerate fissure* 

 (PL LVI. fig. 1 , s). Mesiad of the inner pneumatic foramen is the fossa terminated by 

 the orifices of the vestibular fenestrse (rotunda et ovaUs), which are divided by a short 

 subverticai bar. Below the fenestral fossa begins the Eustachian groove (e). Anterior 

 to the groove leading to the fenestral fossa are two other large oval pneumatic foramina. 

 The tympanic cavity anterior to these is smooth ; but mesially, where it undermines the 

 basisphenoid, it becomes reticulate. The chief part of the floor of the tympanic cavity 

 is membranous. 



At the fore part of the base of the pretympanic plate of the alisphenoid (ib. e) opens 

 the foramen ovale, partly divided into a larger inferior passage for the third, and a smaller 

 upper one for the second, division of the fifth nerve. This foramen is 2 inches 2 lines 

 distant from that of the opposite side, and 8 lines behind the prelacerate fissure* 

 (ib. lo). This, as in most other birds, includes the optic foramen, with those for the 

 transmission of the nerves to the orbit, viz. the sixth and fourth and the anterior divi- 

 sion of the fifth pair. The optic foramen is on the mesial side of the fissure, and is 

 better defined from the orbital nerves and vessels than in most other birds ; a more re- 

 markable peculiarity is the extent of separation of the left from the right prelacerate 

 fissure, the optic foramina being 1 inch 6 lines apart in Dinornis robustus. 



The optic groove, which extends across the fore part of the sella, from one optic 

 foramen to the other, the floor of which is shown in PI. LIV. fig. 1, m, defines the 

 coalesced bases of the orbitosphenoidal neurapophyses. These bases rest upon a pro- 

 longation of bone from the basisphenoid, suddenly narrower than that part, convex 

 transversely, contracting anteriorly, and called in ornithotomy the ' rostrum' or ' sphe- 

 noidal rostrum' (ib. 9). 



If we may extend the more general comparisons of the vertebrate endoskeleton to 

 this part of the base of the skull, we should view this ' rostrum' as the anterior con- 

 tinuation of the series of vertebral elements called ' centrums,' but which have been 

 ossified, like the lower cortical or hypapophysial part of the centrum of the atlas,fr om 

 the capsule of the notochord. The presphenoid, indeed, is only semicylindrical, and 

 offers a close resemblance to the corresponding base of the sacrum, succeeding that 

 which, by its greater breadth and flatness, forms, as a thin floor of bone, the base of 

 the sacral cranium, or neural cavity for the sacral expanse of the myelencephalon, and 

 so closely and instructively repeats the characters of the basioccipital and basisphenoid 

 at the base of the bird's encephalic cranium. To the chambers in which the foremost 

 productions of the myelencephalon expand, this ornithotomical ' rostrum ' exists in the 

 relation of a centrum, both developmentally, connectively, and functionally : the 

 coalesced orbitosphenoids and prefrontals have like relations thereto as neurapophyses. 



' " Fissuia lacera posterior " and " foramen lacerum posterius " of Anthropology. 



' "Fissura lacera anterior," "foramen lacerum anterius," and "fissura sphenoidalis," ibid. 



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