Nevertheless we have yet very much more to learn about the order Ichthyosauria as a whole—whence they came and how they originated; what their nearest kin were among other reptiles; and especially, more about the connecting links between them and terrestrial reptiles. They have, as an order, so isolated a position, are so widely separated from all other reptiles in structure, that they have long been a puzzle to paleontologists. Like the whales and other cetaceans among mammals, we know the ichthyosaurs well in the plenitude of their power and the fulness of their development, but have yet only an imperfect knowledge of their earlier history, and none whatever of their earliest. However, as will be seen farther on, the recent discoveries by Merriam have shed much light on some of the stages of their evolution. So nearly perfectly were all the later ichthyosaurs adapted to their life in the water that it was believed by nearly all paleontologists until about a score of years ago that they had descended directly from fishes. But this belief has been quite abandoned by all, not only because the recent discoveries of the earlier ichthyosaurs have demonstrated a positive increase in the aquatic adaptations of the later forms, but also because a double origin of any type of animal life is quite out of accord with all known facts and principles of paleontology. It is quite possible for animals, in becoming adapted to peculiar environmental and food conditions, to acquire certain resemblances to 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. [Illustration: FIG. 53.—_Baptanodon (Ophthalmosaurus)._ Skull from the side, from above, and from below (after Gilmore): _ang_, angular; _bs_, basisphenoid; _d_, dentary; _fr_, frontal; _j_, jugal; _la_, lacrimal; _mx_, maxilla; _na_, nasal; _oc_, occipital condyle; _p_, palatine; _pa_, parietal; _pm_, premaxilla; _po_, postorbital; _ps_, parasphenoid; _pt_, pterygoid; _pf_, prefrontal; _sa_, surangular; _sp_, splenial; _sq_, squamosal; _st_, supratemporal; _q_, quadrate; _qj_, quadratojugal.] [Illustration: FIG. 54.—Occiput of _Baptanodon (Ophthalmosaurus)_: _pa_, parietal; _soc_, supraoccipital; _sq_, squamosal; _exoc_, exoccipital; _op.o_, paroccipital; _sta_, stapes; _st_, supratemporal; _qu_, quadrate; _qj_, quadratojugal; _pt_, pterygoid; _bs_, basisphenoid; _sag_, surangular; _ag_, angular; _art_, articular; _pra_, prearticular. (After Gilmore.)]