348 PROFESSOR OWEN ON THE GENUS DINORNIS. 
There is not an interorbital septum in Dinornis, as in birds generally; so, if such 
septum can, on any ground, be regarded as ‘ the body of an anterior cranial vertebra,’ 
whether ‘compressed,’ or ‘third,’ or ‘last,’ or ‘most anterior,’' then such ‘body’ 
must be denied to the Dinornis, and the ‘ presphenoid’ be left without general homo- 
logical significance. The truth is, however, that the compressed interorbital septum is 
the result of special ornithic modifications of the general vertebrate type; not, however, 
common to all the class, but an inconstant feature therein. In such low forms as 
Apteryx and Dinornis we see it not: the common vertebrate type is here more closely 
adhered to. The real ‘ body,’ or representative centrum, of the prosencephalic cranial 
vertebra is wanting in no bird; it exemplifies its general homological character more 
clearly where the general vertebrate type is least departed from. Developmentally it is 
the product of the notochordal capsule, and of a part of such extending anteriorly 
beyond the gelatinous contents of the chorda, and it may include only the inferior 
cortical or ‘hypapophysial’ part of the cervical or dorsal centrum, as in the similarly 
produced part of the cranial vertebra in Cetacea: but whether it be specially denomi- 
nated ‘sphenoidal rostrum,’ ‘ presphenoid,’ or ‘ vomer,’ such median inferior parts of 
the floor of the osseous encompassings of the foremost parts and productions of the 
neural axis are in the relation of ‘centrums’ or ‘bodies’ to such ‘ neurapophyses’ 
and ‘ neural spines.’ 
As the basioccipital is coalescent with the exoccipitals and basisphenoid, and this 
again with the alisphenoids, so the forward production of the cranial base is coalescent 
with the orbitosphenoids, which, by their confluent bases, support the optic groove, and 
are at the outer end of this groove pierced by the optic foramina. In Dinornis the 
orbitosphenoids coalesce with the antecedent pair of plates to form the walls of the vast 
rhinal chamber. The presphenoid (Pl. LVI. 9), connate as well as confluent with the 
basisphenoid, presents the usual ornithic or oviparous condition of a long rostrum, but 
here may be said rather to resume the usual subcylindrical shape of the vertebral 
centrums. It is, however, semicylindrical, rounded below from side to side, broad 
and flattened above, where it coalesces with the lamin (14) protecting the most anterior 
developments of the neural axis. It measures from the fore part of the ‘ pterapophyses’ 
3 inches 3 lines, contracting to 6 lines across, near its origin, then gradually expanding 
to a breadth of 8 lines opposite the rhinal apertures, before more gradually contracting 
to its apex. Its free surface is convex transversely and smooth, consisting of a very 
thin plate of bone ; its substance is highly pneumatic, receiving air from the sphenoidal 
communications with, or extensions of, the tympanic cavity. Hollow slender columns 
of bone act as tie-beams on each side, near its confluence with the fore part, connecting 
the lower and lateral walls with the upper wall of this elongate cranial centrum. 
The neurapophysial plates arising from its upper and lateral parts slightly converge 
posteriorly before bending upward and outward to form the combined optic (op) and 
* Melville, ‘Osteology of the Dodo,’ 4to, p. 87 (1848). 
