720 PHYLUM CHORDATA : CLASS REPTILIA 



Examples. — Pareiosaurus, Dicynodon, Elginia. 



Plesiosauria. — Amphibious and marine reptiles represented from the 

 Trias to the Chalk, without exoskeleton, usually with a long neck and 

 short tail. The skull has a single broad temporal arcade, pterygoids 

 meeting in the middle line, fixed quadrates, and a pineal foramen. 

 There are strongly developed pectoral and pelvic girdles. The limbs 

 vary ; in the earlier, more generalised, forms they are adapted for 

 walking on land ; but in the more specialised types they are modified 

 into powerful paddles, Hke those of Chelonia. The nearest affinities 

 are with the Chelonia. Nothosaurus had limbs adapted for progression 

 on land; Plesiosaurus (40 ft. in length) and Pliosaurus were carni- 

 vorous forms adapted to an aquatic life. 



Ichthyosauria. — Large marine carnivorous Reptiles, represented 

 from the Trias to the Chalk, with tapering body like that of a shark, 

 large dorsal and caudal fins, and two pairs of paddle-like limbs. In 

 the paddle the number of digits may be more than five, and the 

 phalanges of each digit are often very numerous. The pectoral arch 

 consists of coracoids, scapulae, clavicles, and a T-shaped episternum. 

 but there is no sternum. The skull has a long tapering rostrum, large 

 orbits, a large parietal foramen, and usually sharp conical teeth in a 

 continuous groove. The vertebras are deeply amphicoelous. There 

 was no dermal armour. The length of the body is sometimes 30 to 

 40 ft. Some species were viviparous. 



Examples. — Ichthyosaurus, Ophthalmosaurus. 



Pythonomorpha. — These strange Cretaceous Reptiles should probably 

 be placed near the Lacertilia and the Rhynchocephalia. They are 

 specially characterised by the enormous elongation of the body, which 

 sometimes reached a length of 75 to 80 ft. The skull is like that of 

 the Monitor among the lizards, but, according to Cope, it also presents 

 affinities with snakes. The body is snake-like, but there are two 

 well-developed pairs of limbs, forming swimming-paddles. All were 

 carnivorous and marine ; the distribution was cosmopolitan. 

 Examples. — Mosasaurus, Clidastes, Liodon, Dolichosaurus. 



Dinosauria. — Terrestrial Reptiles, ranging from the Trias to the 

 Chalk, often very large, and, like Marsupials, specialised in various 

 directions. They were long-necked and long-tailed forms, some 

 bipedal, some quadrupedal. The skull has a superior and an inferior 

 temporal arcade, a fixed quadrate, teeth in sockets, and confined to the 

 margins of the jaws. They exhibit many points of resemblance to 

 Crocodiles and Rhynchocephalia on the one side and to Birds on the 

 other. The pelvis and hind-limbs are particularly avian, e.g. in the 

 tendency to form a tibio-tarsus. Brontosaurus, a gigantic, herbivorous 

 form, nearly 60 ft. in length, was probably amphibious. Atlantosaurus 

 was even larger, the femur measuring over 6 ft. in length. Compso- 

 gnathus, Iguanodon, and Camptosaurus are examples of the "bird- 

 footed " herbivorous Dinosaurs. Compso gnathus only reached a 

 length of 2 ft., and hopped on its hind-legs like a bird. Iguanodon 

 habitually walked on its hind-limbs, and, like several others, had hollow 

 bones ; it reached a height of 15 ft. Of the carnivorous Dinosaurs, 

 Megalosaurus is a good type. The limbs were furnished with powerful 

 claws, and the teeth show much specialisation. 



