PRESIDENT'S ADDRESS. 21 



in distinction to the Ichthyosaurus, whose teeth are anchylosed 

 to the jaws. The teeth of Reptiles like those of Fishes are 

 successional ; new teeth were constantly in progress of develop- 

 ment, at the same time as when the old ones were in course of 

 shedding. The rapid succession of tooth-germs which stamps 

 the impress of decay even before the growth of the new ones is 

 completed, is very apparent in Crocodiles, where three and some- 

 times more than four generations are sheathed one in the other, 

 within the same socket. The Pterosauria, winged Saurians, 

 now extinct ; both limbs modified for aerial flight to each of which 

 was attached a membraneous apparatus similar to that of a Bat. 

 The structure of the skull shows its reptilian character closely 

 approaching that of the Crocodile. The neck, consisting of six 

 vertebrae, is of unusual length in proportion to its body. The 

 orbits, like the Ichthyosaurus, are large ; ,biit there is no trace of 

 bony-sclerotic plates. The bones of the arm and fore-arm are of 

 considerable length, in order to give the membraneous wing 

 sufficient force to raise and move the body in the air. We learn 

 from its fossilized remains that the weight of the body, in 

 proportion to the area of its outspread wings, was greater in 

 Pterosauria than in most Birds, and equal to Bats. The large 

 head and strong neck appear to have been required for the 

 extension and forward direction of the antibrachium (the ulna 

 and radius) by which the centre of gravity was brought further in 

 advance than either in Bird or Bat. The modification for 

 converting the limb into a wing, is confined to the fifth metacarpal 

 and the proximal phalange, which are nearly as thick as the ulna. 

 The other phalanges are similarly elongated. It is of some 

 interest to note the gradual modifications by which the fore-limbs 

 of air-breathing Reptiles adapt them for aquatic, amphibious, 

 arboreal, or aerial-life. Gigantic Pterosauria have been found 

 in the Cretaceous Beds of Europe. Dimorphodon and Rham- 

 phorhyncus in the lower Lias of Lyme Regis and Doratorhyncus 

 in the Purbeck Beds of Swanage. Several species of Pierosauria 

 have been discovered in the Middle Cretaceous Beds of Western 

 Kansas, the first by Marsh in 1870, Pteranodon with an expansion 



