The last order of extinct Reptiles is that of the Dinosauria, comprising a group of very remarkable Reptiles, which are in some respects intermediate in their characters between the Struthious Birds and the typical Reptiles; whilst they have been supposed to have affinities to the Pachydermatous Mammals. Most of the Dinosauria were of gigantic size, and the order is defined by the following characters:

The skin was sometimes naked, sometimes furnished with a well- developed exoskeleton, consisting of bony shields, much resembling those of the Crocodiles. A few of the anterior vertebra were opisthocoelous, the remainder having flat or slightly biconcave bodies. The anterior trunk-ribs were double-headed. The teeth were confined to the jaws and implanted in distinct sockets. There were always two pairs of limbs, and these were strong, furnished with claws, and adapted for terrestrial progression. In some cases the fore-limbs were very small in propor-tion to the size of the hind-limbs. No clavicles have been discovered.

The teeth are sometimes implanted in distinct sockets, and they are never anchylosed with the jaws. The ischium and pubes are much elongated; the inner wall of the acetabulum is formed by membrane; the tibia has its proximal end prolonged anteriorly into a strong crest; and the astragalus is bird-like (Huxley).

The most remarkable points in the organisation of the Dinosauria are connected with the structure of the pelvis and hind-limb, the characters of which, as pointed out by Huxley, approximate to those of the same parts in the Birds, and especially in the Struthious Birds. This approximation is especially seen in the prolongation of the ilium in front of the acetabulum (fig. 318), the elongation and slenderness of form of the ischium, and the slenderness of the pubes. The astragalus is like that of a bird, and in some cases appears to have become anchylosed with the distal end of the tibia. The metatarsal bones, however, remain distinct, and are not anchylosed with any of the tarsal bones to form a "tarso-metatarsus."

Fig. 318.   Leg of Deinosaur. il Ilium; is Ischium ; f Femur ; t Tibia ; .s Fibula ; as Astragalus; ca Calcaneum ; m Metatarsus. (After Huxley.)

Fig. 318. - Leg of Deinosaur. il Ilium; is Ischium ; f Femur ; t Tibia ; .s Fibula ; as Astragalus; ca Calcaneum ; m Metatarsus. (After Huxley.)

The most familiar examples of the Dinosauria are Megalosaurns and Iguanodon.

Megalosaurus is a gigantic Oolitic Reptile, which occurs also in the Cretaceous series (Weald Clay). Its length has been estimated at between forty and fifty feet, the femur and tibia each measuring about three feet in length. As the head of the femur is set on nearly at right angles with the shaft, whilst all the long bones contain large medullary cavities, there can be no doubt but that Megalosaurus was terrestrial in its habits. That it was carnivorous and destructive in the highest degree is shown by the powerful, pointed, and trenchant teeth.

The Iguanodon is mainly, if not exclusively, Cretaceous, being especially characteristic of the great delta-deposit of the Wealden. The length of the Iguanodon has been estimated as being probably from fifty to sixty feet; and from the close resemblance of its teeth to those of the living Iguanas, there is little doubt that it was herbivorous and not carnivorous. The femur of a large Iguanodon measures from four to five feet in length, with a circumference of twenty-two inches in its smallest part. From the disproportionately small size of the fore-limbs, and from the occurrence of pairs of gigantic three-toed footsteps in the same beds, it has been concluded, with much probability, that Iguanodon, in spite of its enormous bulk, must have walked temporarily or permanently upon its hind-legs, thus coming to present a most marked and striking affinity to the Birds.

The most remarkable, however, of the Dinosauria, is the little Comp-sognathus longipes, from the Lithographic Slate of Solenhofen, referred to this order by Professor Huxley. This Reptile is not remarkable for its size, which does not seem to have been much more than two feet, but for the singular affinities which it exhibits to the true Birds. The head of Compsognathus was furnished with toothed jaws, and supported upon a long and slender neck. The fore-limbs were very short, but the hind-limbs were long and like those of Birds. The proximal portion of the tarsus resembled that of Birds in being anchylosed to the lower end of the tibia ; but the distal portion of the tarsus - unlike that of Birds - was free, and was not anchylosed with the metatarsus. Huxley concludes that "it is impossible to look at the conformation of this strange Reptile, and to doubt that it hopped or walked in an erect or semi-erect position, after the manner of a Bird, to which its long neck, slight head, and small anterior limbs must have given it an extraordinary resemblance."

The researches of Professor Phillips, further, have now shown that the gigantic Cetiosaurus of the Oolitic and Cretaceous rocks, formerly referred to the Crocodilia, is truly a Deinosaur. Its total length is estimated at probably not less than sixty or seventy feet.