The vertebrae are either biconcave or procoelous. The premaxillae are small, and the anterior nares are united and placed far forwards. The posterior nares lie far back, the palatines and in living genera the pterygoids, meeting in the middle line, and giving rise to a closed palate. The supratemporal fossae are surrounded by bone on all sides, and the parietals, and often also the frontals are united. There are no clavicles. The suborder includes the genera _Crocodilus_, _Alligator_, _Garialis_ and others living and extinct. _Order 9._ PTEROSAURIA[88]. These animals, called also the pterodactyles or Ornithosauria, are a group of extinct reptiles, whose structure has been greatly modified from the ordinary reptilian type for the purpose of flight. The skin was naked and they vary greatly in size and in the length of the tail. The vertebrae and limb bones are pneumatic just as in birds. The presacral vertebrae are procoelous and have their neural arches firmly united to the centra. The neck is long, the caudal vertebrae are amphicoelous, and from three to five vertebrae are fused together in the sacral region. The skull is large and somewhat bird-like, the facial portion being much drawn out anteriorly, and the sutures being obliterated. It resembles that of other reptiles in having large supratemporal fossae; large pre-orbital vacuities also occur. The jaws may be toothed or toothless, and the teeth, when present, are imbedded in separate sockets. The premaxillae are large, and the quadrate is firmly attached to the skull. The rami of the mandible are united at the symphysis, and there is an ossified ring in the sclerotic. The occurrence of a postfrontal and its union with the jugal behind the orbit, are characteristic reptilian features. The ribs have capitula and tubercula, and sternal and abdominal ribs occur. The sternum has a well-developed keel, and the scapula and coracoid are large and bird-like. There are no clavicles or interclavicle. The anterior limbs are modified to form wings by the great elongation of the fifth digit, to which a membrane was attached. The second, third and fourth digits are clawed and are not elongated in the way that they are in bats. The pollex, if present at all, is quite vestigial. The pelvis is weak and small, and though the ilia are produced both in front of and behind the acetabula, in other features the pelvis is not bird-like. The ischia are short and wide, and the pubes are represented only by the pre-pubes. The posterior limbs are small and the fibula is much reduced. The pes is quite reptilian in type, and has five separate slender metatarsals. The two best known genera are _Pterodactylus_, in which the tail is short, and _Rhamphorhynchus_, in which it is long. The Pterosauria are found throughout the Jurassic and Cretaceous formations in both Europe and North America. FOOTNOTES: [65] According to Baur a distinct epi-otic is not recognisable in the reptilian skull.