954 PRINCIPLES OF STRATIGRAPHY 



Typical marine Crocodiles occur in the Jurassic and Comanchic 

 (Mesosuchia), while in the Cretacic-Tertiary and modern times 

 these crocodiles ( Eusuchia) again lived chiefly in fresh water and 

 on the land. They include both long-snouted (longirostral) and 

 broad-snouted (brevirostral) forms, the latter comprising the alli- 

 gators. 



The Dinosauria were long-necked and long-tailed reptiles with 

 limbs adapted for support of the body. The earliest species were 

 Triassic, the latest Cretacic. A bony exoskeleton was developed in 

 some forms, consisting of isolated bony plates or spines, or of 

 interlocking scutes forming a continuous shield. Most dinosaurs, 

 however, were naked or covered by scales. The skull of most forms 

 was extremely small in proportion to the body, while the legs in 

 many cases were exceedingly massive. 



The Pterosauria, or winged lizards, ranged from the Trias to the 

 Cretacic, and their whole organization was adapted to an aerial ex- 

 istence. They ranged from the size of a sparrow to forms which had 

 a spread of wing of nearly six meters. The skull was bird-like and 

 generally fitted with sharp, conical teeth, mostly long and sharply 

 pointed in front (Pteranodon, Nyctodactylus, Ramphorhynchus), 

 but sometimes blunt (Dimorphoden). The neck and tail were gen- 

 erally long. The fifth digit of the hand consisted of four enor- 

 mously elongated phalanges, which were turned backward to 

 support the wing membrane. Three families are known : Rham- 

 phorhynchidce (Jurassic), Pterodactylidce (Upper Jurassic and 

 Cretacic), and Ornithocheiridcs (Pteranodon, etc.) Cretacic. 



Aves. 



The birds form a homogeneous and circumscribed class derived 

 from the reptiles and partaking of their character in the Jurassic 

 and Cretacic, where teeth and a vertebrated tail still existed. The 

 exoskeleton consists of feathers, horny coverings for the beak, 

 claws, etc. The endoskeleton is compact but light, the bones being 

 permeated by air-cavities with thin but dense-textured walls, rich in 

 calcium phosphate. The vertebrae have peculiar saddle-shaped ar- 

 ticulations which allow great freedom of movement. The bones of 

 the forearm are modified into wings. The oldest bird, Arch?eop- 

 teryx of the Jurassic, had its jaws provided with conical teeth like 

 those of reptiles, and its vertebral column had about 20 caudal verte- 

 bra. The Odontolcae (with Hesperornis) and Odontoniuv (with 

 Ichthyornis) also had toothed jaws, but other birds were free from 



