The skin is in most forms at any rate unprovided with dermal scutes. The vertebrae may be with or without zygosphenes and zygantra. The skull resembles that of lizards, having an interparietal foramen, and a cranial cavity open in front. The squamosal takes part in the formation of the cranial wall, and the quadrate articulates with the squamosal, not as in Lacertilia with the exoccipital. There are large supratemporal fossae, bounded below by supratemporal arcades. The teeth are large and acrodont, and occur on the pterygoids as well as on the jaws. The two rami of the mandible are united by ligament only. Pectoral and pelvic girdles are present, but clavicles are wanting, and the pelvis is not as a rule united to any sacrum. The limbs are pentedactylate, and are adapted for swimming, while all the limb bones except the phalanges are relatively very short. The number of phalanges is not increased beyond the normal, and they articulate with one another by flat surfaces. The terminal phalanges are without claws. _Order 7._ DINOSAURIA[84]. The extinct reptiles comprising this order were all terrestrial, and include the largest terrestrial animals known. They vary greatly in size and in the structure of the limbs, some approach close to the type of structure met with in birds, others are allied to crocodiles. Passing to the more detailed characters:--there is sometimes a well-developed exoskeleton having the form of bony plates or spines. The vertebrae may be solid or their centra may be hollowed internally; their surfaces may be flat, biconcave or opisthocoelous. The sacrum is composed of from two to six vertebrae. As regards the skull, the quadrate is large and fixed, and supratemporal and infratemporal fossae bounded by bone occur. The teeth are more or less laterally compressed, and often have serrated edges; they may be placed in distinct sockets or in a continuous groove. The ribs have capitula and tubercula, and sternal ribs often occur. The scapula is very large, the coracoid small, and there is no precoracoid, or T-shaped interclavicle. Clavicles are only known in a few cases. In the pelvis the ilium is elongated both in front of, and behind, the acetabulum, sometimes the pre-pubis, sometimes the post-pubis is the better developed. The anterior limbs are shorter than the posterior, and the long bones are sometimes solid, sometimes hollow. There are three well-marked suborders of the Dinosauria. _Suborder (1)._ SAUROPODA[85]. The reptiles belonging to this group were probably quadrupedal and herbivorous. They have the cervical and anterior trunk vertebrae opisthocoelous, while the posterior vertebrae are biconcave; all the presacral, and sometimes the sacral vertebrae are hollowed internally. The teeth are spatulate and without serrated edges, they are always planted in distinct sockets, and some of them are borne by the premaxillae. [Illustration FIG. 35. RESTORED SKELETON OF _Ceratosaurus nasicornis_. × 1/30. (After Marsh.)