The caudal vertebræ differ in many ways from other animals. They have neither transverse processes, neural spines, hypapophyses or hæmapophyses. In the persistence of the neural arch down the tail they resemble reptiles and birds rather than mammals, in which nothing but the centrum persists to the end of the tail. The vertebræ are furnished with vertebral arteries which run through the neural arch parallel to the neural canal, in exactly the same position as do the vertebral arteries in the neck-vertebræ of the Llama.
[THE BONES OF THE HEAD.]
The skull of Dimorphodon differs in form and in many important details of structure from that of Rhamphorhynchus; and both of these types of skull are strikingly unlike that of the short-tailed animals named Pterodactyle. Hence, as it will be shown that the Cretaceous fossils of this class belong to very distinct new genera, there is no reason for assigning to them by anticipation any class of cranial structures. The cranium of this type of animal has never been critically described, and for all that is yet known to the contrary Pterodactyles may differ between themselves as much as birds or mammals. Their affinities have been unknown. Therefore, before describing bones it may be desirable to state the grounds on which the several specimens are referred to the Ornithosauria. The fossils on which this section of the memoir is founded are, the basi-occipital and basi-temporal bones, the anterior portion of a cranium, the back parts of four crania, facial bones, and the quadrate and quadrato-jugal.
The crania are all no larger than that of the Heron; though from the Greensand are bones and jaws indicating Pterodactyles both smaller and larger. The skulls are mostly remarkable for wanting both basi-occipital and basi-temporal bones. And the specimen of basi-temporal and basi-occipital corresponds posteriorly with the Pterodactyle atlas, anteriorly with these crania; it is hence concluded to have belonged to a similar animal. Being relatively twice as large, it indicates that in these animals the basi-occipital condyle was proportionally larger than in known birds; and that animals of a cognate kind had skulls probably twice the size of these. The anterior basal part of the hinder sphenoid terminates in a remarkable triangular surface, with two perforations, which are separated by a median ridge. Almost entirely corresponding with this is the basal surface of the anterior part of a cranium, fractured in front of the pituitary fossa. Therefore, and as it indicates a similar capacity of brain, it is regarded as belonging to the same kind of animal as the others ; but being five times the size, it must, if the proportions of the Heron were preserved, have been part of a head a yard long.
Now, as there is no other animal with the same texture of bone, or exhibiting with high organization the same diversity of size, these cranial fragments are referred to the jaws and bones of Pterodactyle. So marked are their structures that many quarry-men refer vertebrate fossils to their several orders with almost as much accuracy as would a practised anatomist.
| Case. | Comp. | Tablet. |
| J | c | 7 |
Basi-occipital and Basi-temporal.
[Pl. 11.]
Basi-occipital, Owen, Sup. Cret. Rep. p. 6, T. 1, figs. 11, 12, 13.
This bone was not found associated with any set of fossils that would induce us to refer it to one species more than to another. Its Ornithosaurian character was probable; and Prof. Owen described it in his last memoir on the Greensand Pterodactyles.