The frontal-bone forms the highest part of the skull, and is similar to that of birds. The principal frontal is double, and forms the upper and hind part of the cavity for the eye, and covered the greater part of the large brain, composed of two hemispheres, in which Oken long ago saw a similarity to the higher animals. The arched form of the back part of the skull is bird-like. The double parietal adjoins the principal frontal, and is conditioned like the parietal in birds. The supra-occipital is single as in birds, expanded, and forms the part of the skull which extends furthest back. From the form of the back part of the skull it may be concluded that the foramen magnum was situated as in birds, and that the head and neck were moved as in birds, and not as in reptiles and mammals.
The temporal bone rests upon the parietal and frontal, and forms much of the temporal foss. Its anterior border does not appear to enter into the margin of the orbital cavity as in birds, but seems to be replaced by the post-frontal, which resembles that of the Chameleon. Its hindmost branch, which can hardly be supposed to be the jugal, forms the outer boundary of the temporal foss by uniting with a process which is probably part of the mastoid. A similar closing of the cavity for the temporal muscles is also to be found in birds. The jugal and maxillary do not follow the bird type. The jugal consists of a single bone which forms the greater part of the anterior and inferior boundary of the cavity of the eye, which is surrounded with bones, as in Dragons and Iguana. In those birds in which the cavity of the eye is surrounded with bones the jugal does not enter into it. As in lizards, at its upper end the jugal is commonly connected with the lachrymal, which bone is like that of a bird. A bone, which appears to be the pre-frontal, enters into the back of the nasal aperture.
The nostril is double and often of large size.
The perforation in the skull between the orbit and nares is bird-like.
The quadrate bone is not quadratic as in birds, but cylindrical and shaft-like, as in the Chameleon. The articulation of the quadrate with the lower jaw is placed further forward than in birds and reptiles. The lower jaw, but for the teeth, has great similarity with that of a bird. Among reptiles its nearest resemblance is with Chameleons and Turtles. The hyoid is more bird-like than reptile-like.
Ribs and vertebræ.
It is uncertain whether the Pterodactyle had lumbar vertebræ. If they are wanting, therein the animals resemble birds, of which we are reminded in the short and stiff back and moveable neck. Pterodactyles possess a smaller number of neck-vertebræ and a larger number of back-vertebræ than birds. The long neck-vertebræ are paralleled by those of water-birds, by the Giraffe, the Camel, Protosaurus and Tanystrophæus. There are 7 cervical vertebræ, the 1st very short, 2nd not longer, but rather shorter than those which follow. There are in Pterodactyles from 12 to 16 dorsal vertebræ, while birds have never more than 11. It is not certain whether all Pterodactyles have an os sacrum; most have it, and therein resemble Mammals, Birds, and some fossil Saurians. In Pterodactylus dubius and P. grandipelvis and P. Kochi there are 5 or 6 vertebræ in the sacrum. In birds the sacral vertebræ vary from 5 to 22; in bats the number is from 5 to 6.
The short tails of Pterodactyles are more like those of mammals than birds; they include from 10 to 15 tail-vertebræ. In birds there are from 6 to 10 tail-vertebræ. Rhamphorhynchus has 38-40 tail-vertebræ, secured between thread-bones like those in the tail of rats.
The dorsal ribs are reptile-like. In herbivorous mammals and birds they are broader. A few species have the first pair of ribs large. The abdominal ribs belong neither to birds nor mammals, but are reptilian. In Rhamphorhynchus Gemmingi there are 6 pairs of sternal ribs.