PROFESSOR OWEN ON THE GENUS DINORNIS. 355 
the end of the tuberosity 1 inch 4 lines; the breadth of the tract is 1 inch 4 line: in 
the centre of the tract is a small venous foramen. ‘The tuberosities bend forward and 
inward ; each is indented by an oblique channel; and from each a ridge continuing the 
convergence is lost after 5 lines course upon the fore part of the basisphenoid. This 
curves upward and contracts to a median ridge slightly produced, as a compressed 
process, projecting about 2 lines forward freely below the base of the presphenoid 
(Pl. XL. fig. 3, 9). The occipital foramen (Pl. XLI. fig. 1, 0) is vertically elongate, with 
a small process on each side, projecting inward and forward from the junction of the 
lower with the middle third, as in Aptornis otidiformis!. he vertical diameter of the 
foramen is 7 lines, the transverse one 53 lines, the foramen being relatively smaller than 
in Aptornis otidiformis. As in that species, the occipital surface, as it rises from the 
foramen magnum, slopes forward to the superoccipital ridge (Pl. XL. fig. 2, 3). 
From the under and inner base of the paroccipital an irregular ridge or bar of bone 
(Pl. XL. figs. 1 & 3, 4’) passes downward and inward, forming the outer side of the 
vagal fossa, and bending forward into and abutting against the smooth deep channel 
outside the descending basicranial tract (1-5), where it terminates like an adherent 
process, with a rough tuberous ending. It was to the left of these productions (Trans. 
Zool. Soc. vol. iii. pl. 52. fig. 6, 4’) of the paroccipital, which might be called “ styloid 
processes,” that the proximal element of the hyoid arch (stylohyal, ib. 38) was anchy- 
losed in the skull of Aptornis otidiformis: this is significant of the arbitrariness of the 
ascription of the tympanic or quadrate bone to that arch. The hind part of the base of 
the alisphenoid is more produced and tuberous outside the end of the hyoid process of 
the paroccipital in Aptornis defossor than in Apt. otidiformis. Between this process 
and the expanded base of the alisphenoid there is a groove-like extension of the 
tympanic cavity. 
The alisphenoid expansion is pneumatic; in advance of that swelling are two wide 
pneumatic openings; and two lines in advance of these is the foramen ovale. 
The mastoid in mammals is characterized by its early ossification, the centre or 
centres appearing in the primordial or protocranial cartilage containing the acoustic 
vesicle. In this developmental relation Cuvier’s “temporal” in birds agrees with the 
mammalian mastoid. Mr. Parker admits that the mastoids are already ossified at the 
“time that the parietals are small ovoid patches;” but he cannot apparently bring 
himself to state that his “squama temporis” in the chick is ossified in and from the 
protocranial cartilage, including the labyrinth. The “ squama temporis” in the human 
embryo is ossified in a membranous basis, like the parietal; the base of the zygoma 
alone shows cartilage. The condition of the mammalian squamosal in Monotremes, 
in which it is almost reduced to its zygomatic part, shows well the homologous 
bone in birds. The mastoid, connate, as usual in birds, with the petrosal, here joins 
the alisphenoid, pushing inward, between the pneumatic vacuities and the canal for 
1 Trans. Zool. Soc. vol. iii. pl. 52. fig. 4, 0. 
302 
