I may recall the circumstance that the discovery of skeletons of fossil animals has occasionally followed upon the interpretation of a single fragment, from which the animal has been well defined, and sometimes accurately drawn, before it was ever seen. So I propose, before drawing any conclusions from the skeletons in the entirety of their construction, to examine them bone by bone, and region by region, for evidence that will manifest the nature of this brood of Dragons. Their living kindred, and perhaps their extinct allies, assembled as a jury, may be able to determine whether resemblances exist between them, and whether such similarity between the bones as exists is a common inheritance, or is a common acquisition due to similar ways of life, and no evidence of the grade of the organism among vertebrate animals.
The bones of these Ornithosaurs, when found isolated, first have to be separated from the organisms with which they are associated and mixed in the geological strata. This discrimination is accomplished in the first instance by means of the texture of the surface. The density and polish of the bones is even more marked than in the bones of birds, and is usually associated with a peculiar thinness of substance of the bone, which is comparable to the condition in a bird, though usually a little stouter, so that the bones resist crushing better. Pterodactyle bones in many instances are recognised by their straightness and comparatively uniform dimensions, due to the exceptional number of long bones which enter into the structure of the wing as compared with birds. When the bones are unerringly determined as Ornithosaurian, they are placed side by side with all the bones which are most like them, till, judged by the standard of the structures of living animals, the fossil is found to show a composite construction as though it were not one animal but many, while its individual bones often show equally composite characters, as though parts of the corresponding bone in several animals had been cunningly fitted together and moulded into shape.
THE PLAN OF THE HEAD IN ORNITHOSAURS
The head is always the most instructive part of an animal. It is less than an inch long in the small Solenhofen skeleton named Pterodactylus brevirostris, and is said to be three feet nine inches long in the toothless Pterodactyle Ornithostoma from the Chalk of Kansas. Most of these animals have a long, slender, conical form of head, tapering to the point like the beak of a Heron, forming a long triangle when seen from above or from the side. Sometimes the head is depressed in front, with the beak flattened or rounded as in a Duck or Goose, and occasionally in some Wealden and Greensand species the jaws are truncated in front in a massive snout quite unlike any bird. The back of the head is sometimes rounded as among birds, showing a smooth pear-shaped posterior convexity in the region of the brain. Sometimes the back of the head is square and vertical or oblique. Occasionally a great crest of cellular tissue is extended backward from above the brain case over the spines of the neck bones.
There are always from two to four lateral openings in the skull. First, the nostril is nearest to the extremity of the beak. Secondly, the orbits of the eyes are placed far backward. These two openings are always present. The nostril may incline upward. The orbits of the eyes are usually lateral, though their upper borders sometimes closely approximate, as in the woodpecker-like types from the Solenhofen Slate named Pterodactylus Kochi, now separated as another genus. In most genera there is an opening in the side of the head, between the eye hole and the nostril, known as the antorbital vacuity; and another opening, which is variable in size and known as the temporal vacuity, is placed behind the eye. The former is common in the skulls of birds, the latter is absent from all birds and found in many reptiles.
The palate is usually imperfectly seen, but English and American specimens have shown that it has much in common with the palate in birds, though it varies greatly in form of the bones in representatives from the Lias, Oolites, and Cretaceous rocks.
From the scientific aspect the relative size of the head, its form, and the positions and dimensions of its apertures and processes, are of little importance in comparison with its plan of construction, as evidenced by the positions and relations to each other of the bones of which it is formed. There usually is some difficulty in stating the limits of the bones of the skull, because in Pterodactyles, as among birds, they usually blend together, so that in the adult animal the sutures between the bones are commonly obliterated.
Bones have relations to each other and places in the head which can only change as the organs with which they are associated change their positions. No matter what the position of a nostril may be—at the extremity of a long snout, as in an ant-eater, or far back at the top of the head in a porpoise, or at the side of the head in a bird—it is always bordered by substantially the same bones, which vary in length and size with the changing place of the nostril and the form of the head. Every region of the head is defined by this method of construction; so that eye holes and nose holes, brain case and jaw bones, palate and teeth, beak, and back of the skull are all instructive to those who seek out the life-history of these animals. We may briefly examine the head of an Ornithosaurian.
BONES ABOUT THE NOSTRIL
No matter what its form may be, the head of an Ornithosaur always terminates in front in a single bone called the intermaxillary. It sends a bar of bone backward above the visible nostrils, between them; and a bar on each side forms the margin of the jaw in which teeth are implanted. The bone varies in depth, length, sharpness, bluntness, slenderness, and massiveness. As the bone becomes long the jaw is compressed from side to side, and the openings of the nostrils are removed backward to an increasing distance from the extremity of the beak.