But the largest Pterodactyls hail, like so many other big things, from America. Professor Marsh tells us of monsters in his famous collection with a spread of wings of twenty to twenty-five feet! These large forms had no teeth in their jaws, and their skulls are of a peculiar form. The long-pointed jaws were probably sheathed in horn during life, as in birds (see Fig. 36). According to Marsh, these toothless forms (which he calls Pteranodonts) were mostly of gigantic size. With regard to their food it is almost vain to speculate; but if they _did_ prey upon fishes, they must have had a capacious mouth and gullet, and must have swallowed their prey whole, after the fashion of pelicans. But we doubt if they had the peculiar pouch possessed by those birds. In the absence of more complete accounts of the large forms the artist has only attempted to restore the small ones. (See Plate XII., showing four different kinds.) [Illustration: Plate XII. GROUP OF SMALL FLYING DRAGONS, OR PTERODACTYLS. _Rhamphorhynchus phyllurus._ _Pterodactylus crassirostris. Dimorphodon macronyx._ _Pterodactylus spectabilis._] Whether Pterodactyls were cold-blooded or warm-blooded is a question on which the authorities are not agreed. Professor Owen argued from the absence of feathers that they could not have been warm-blooded. But, in spite of this great authority, who has defended his opinion somewhat strongly, there are others who argue that the amount of work involved in sustaining a Pterodactyl in the air make it highly probable that it was warm-blooded. The absence of feathers to retain the heat of the body need not be regarded as conclusive, for bats are warm-blooded animals, and in their case the heat of the body is retained by a slight downy covering to the skin. Such a covering may have protected the bodies of Pterodactyls, and we could not expect to see any trace of it in the Bavarian specimen of Rhamphorhynchus referred to above. An important fact bearing on this question is the discovery of perforations in the bones of these animals very similar to those seen in birds. Now, birds have a wonderful system of respiration, or breathing. The air they breathe passes, not into their lungs only, but penetrates to the remotest parts of their system, filling their very bones with life, and endowing them with activity and animation adapted to their active aërial existence. It may, therefore, be argued that Pterodactyls breathed much in the same way; that their bones, too, were supplied with air by an elaborate system of air-sacs, and that they had lungs like those of birds. We cannot, however, stop there, but are led on by physiological reasoning to conclude that the circulation of the blood must have been rapid, and that the heart was like that of birds and mammals, four-celled. It would therefore follow--since birds and mammals are warm-blooded--that Pterodactyls were also. Such, at least, is the view of Professor H. G.