114 



UATER REPTILES OF THE PAST AND PRESENT 



other animals, but quite impossible for them to acquire their actual 

 structure. The ichthyosaurs are true reptiles, and all reptiles 

 must have had a common origin. 



We are sometimes in doubt, however, as to whether characters 

 resembling those of other animals are really acquired as adaptations 

 to peculiar environments, that is, parallel, convergent, or homo- 

 plastic characters, or whether they are due to heredity from remote 

 ancestors. The reptilian characters of the ichthyosaurs, however, 

 are so emphatic that they can only be ascribed to heredity. Ichthy- 

 osaurs are as truly reptilian 

 as crocodiles or snakes, not- 

 withstanding their fish-like 

 form and habits. The ich- 

 thyosaur ancestors were once 

 truly land reptiles — of that 

 we are as sure as we well 

 can be. Some have thought 

 that those ancestors were the 

 primitive Rhynchocephalia, 

 but most are now convinced 

 that they were among the 

 most primitive of reptiles, a 

 branch probably from the 

 cotylosaurs or cotylosaurian 

 ancestors. Probablv of all 

 the extinct forms that we 

 know the Proganosauria 

 come the nearest; indeed it 

 is not impossible that they may have been the actual forbears of 

 the ichthyosaurs. 



The ichthyosaurs varied in length from two to thirty feet, but 

 the different species, especially all the later ones, resembled each 

 other pretty closely in shape; the beak was more slender in some 

 than in others, and the shapes of the fins and paddles varied not a 

 little, as we shall see. The jaws were long and slendet^ provided 

 with numerous rather small but sharp and recurved teeth, espe- 

 cially well fitted for the seizure and retention of slippery prey. The 



Fig. 54. — Occiput of Bapianodon (Opli- 

 tlialmosaunis): pa, parietal; soc, supraoccip- 

 ital; sq, squamosal; exoc, exoccipital; op.o, 

 paroccipital; sta, stapes; st, supratemporal; 

 gi(, quadrate; qj, quadratojugal; pi, ptery- 

 goid; bs, basisphenoid; sag, surangular; ag, 

 angular; art, articular; pra. prcarticular. 

 (After Gilmore.) 



