It is 7/16 of an inch long; posteriorly 9/16 wide, nearly 6/16 of an inch high anteriorly. It terminates in front superiorly in an elevated transverse ridge.
On removing the matrix, the anterior surface of this occipital bone was found to be concave; yet as nothing but cancellous structure is seen it may be but imperfectly ossified or more probably, imperfectly preserved. And the bottom of this cup expands forward in a thin sheet of bone a quarter of an inch long and half an inch wide, which on the under side is continuous with the base of the condyle.
On each side of this floor and partly extending in front of it, and below it, is an irregular piece of bone, half an inch long, resembling anterior zygapophyses of cervical vertebræ.
Though in most vertebrates the basi-occipital enters into the basal floor of the skull, the median bones are either so placed that they rest one upon another from before backwards or abut against one another nearly perpendicular, so that the basi-sphenoid comes commonly to underlap and partly hide the basi-occipital. Nowhere among Amphibia or Reptilia do I know of the reverse position occurring. In some fishes there is an approach to it. Thus a slight anterior bony expansion of the basi-occipital in the Cod fits partly into a horizontal slit in the basi-sphenoid[A]. In the Carp the basi-occipital has a spathulate basal expansion like that of Pterodactyle, but it is underlapped by the basi-sphenoid[S]. In some mammals the under side of the basi-occipital extends further forward than does the neural side, as for example in the Sheep and Goat; while in a few others, as in the Walrus, the reverse positions obtain.
[Footnote S: Parasphenoid of Prof. Huxley.]
But it is among Birds that the structure described in Pterodactyle is evident and characteristic. For although the bony plate under the sphenoid,--Mr Parker's basi-temporals,--is mostly anchylosed to the bones about it, and less with the occipital than with others, its position and relations are quite the same as those of the expanded flap of this Pterodactyle basi-occipital. Therefore it is identified with the basi-temporal bones.
Case. Comp. Tablet. Specimen. =J= _c_ 8 1
This fossil is an inch high, rather wider, and half an inch long. It well shows the bones at the back of the skull, the basi-cranial bones, and the bones posterior to the frontals, which roof in the Cranium. There are in it striking resemblances to the back of the skull of some Natatores, as the Grannet and Cormorant, and of some Grallatores as the Heron, and Gallinaceous birds as the Cock.