28 REPTILES 



as the Lower Lias ; but as some of the Cretaceous more special- 

 ised species were equally large, the dying-out of the group 

 seems to have been sudden, as there is no sign of decadence 

 or degeneration. The cause of this apparently sudden exter- 

 mination is altogether beyond our ken ! 



The geological history of the plesiosaurs (Sauropterygia) is 

 very similar to that of the ichthyosaurs, except that the Triassic 

 types were smaller and more nearly akin to terrestrial forms, 

 and that the largest species occurred in the Upper Jurassic and 

 Cretaceous periods, although some of those from the Lias were 

 gigantic. The small group of placodonts (Placodontia) appears 

 to have been solely Triassic. 



Chelonians, as represented by Proganochelys and Chclyther- 

 i/nu, date from the Upper Trias, but these forms are as typically 

 chelonian as is any modern tortoise, so that we have evidently 

 to go further back before the order can be traced to the ancestral 

 stock. From the Trias chelonians appear to have steadily in- 

 creased in numbers to the present day, when they form the 

 second largest reptilian order. The Chelonia of the Jurassic 

 strata belong to a generalised group known as the Amphichel- 

 ydia, from which appear to have diverged the modern Crypto- 

 dira and Pleurodira, of which the latter is now on the wane. 

 The origin of the soft-tortoises, or Trionychoidea, is unknown, 

 but they were abundant in the early Tertiary, and also occur in 

 the Upper Cretaceous. True marine turtles (Chc/onidcc) also 

 date from the Upper Cretaceous, and were preceded by ancestral 

 forms in the Upper Jurassic. Upon the question whether the 

 Triassic Psephoderma is really chelonian or not, largely depends 

 the problem as to the nature of the relationship between the true 

 turtles {Chclonidce) and the leathery turtles {Dermochelyidtv) of 

 our modern seas. 



The pterodactyles, or Ornithosauria, according to our 

 present information, date from the Lower Lias and continued 

 right through the overlying Mesozoic formations. Since, 

 however, their earliest known representative — Dimorphodon 

 macronyx of the Lower Lias of Dorsetshire — is in all respects a 

 typical and fully evolved member of the group, it can scarcely 

 be doubted that their origin must be looked for at an earlier 

 epoch of the earth's history. Beyond the fact that some of 

 the more specialised forms of the later formations have lost 



