GREAT STEPS IN ORGANIC EVOLUTION 839 



indication of horny scales about the front and base of the fore- 

 paddles, but otherwise the fish-lizards seem to have been smooth- 

 skinned, just as the mammalian cetaceans at a much later date 

 became. 



An interesting point is that the tail had a large bilobed vertical 

 fin like that of a shark, not a pair of horizontal flukes as in whales 

 and dolphins ; but whereas the end of the vertebral column of a shark 

 is prolonged into the upper half of the unsymmetrical tail, it is 

 prolonged into the lower half of the also unsymmetrical tail of the 

 Ichthyosaur. When a shark strikes with its tail — upper half the 

 larger — the movement tends to depress the head, unless, of course, 

 the shark has turned upside down, as it often does, at the surface. 

 But as the stronger half of the fish-lizard's tail is the lower one, into 

 which the end of the vertebral column is abruptly bent, the stroke 

 must have tended to bring the head up — and this, as one of our old 

 professors first pointed out, would tend to lift the head and its 

 nostrils upwards at the surface, an obvious adaptation for a lung- 

 breathing reptile. 



The skull is drawn out into a long snout, which is mostly made of 

 the bones that become exaggerated to frame the beak of a bird, 

 namely, the premaxillae. As in all birds, except the kiwi (Apteryx), 

 the nostrils are far back. The orbits for the eyes are remarkably 

 large, and we see that the front of the eyeball was protected by a 

 ring of bone — the sclerotic ring. But this occurred in many of the 

 extinct reptiles, and formed part of the legacy that certain Dinosaurs 

 handed on to birds, these surprising scions of an adventurous, 

 though markedly pedestrian stock. 



On the top of the Ichthyosaur's skull there is a well-marked hole 

 — the parietal foramen — which lodged the pineal body, that strange, 

 upward-looking median eye, represented in some degree in all 

 Vertebrates from lamprey to man, but showing only in a few animals 

 to-day, such as the New Zealand "lizard" or Sphenodon, distinct 

 traces of optic structure. It is the organ about which we are always 

 told that Descartes regarded it as the seat of the soul. If so, it was 

 very generous of him, for it means including as besouled the lamprey 

 and the tadpole, the slow-worm and the Ichthyosaur, and thousands 

 of more ordinary Vertebrates. 



In most Ichthyosaurs the conical teeth, suggesting those of some 

 of the toothed whales, are placed in a continuous groove. They were 

 evidently suited for seizing and gripping slippery prey rather than 

 for chewing. From the shape of the fossilised alimentary debris 

 (coprolites) it seems clear that the Ichthyosaurs had a "spiral valve" 

 like that of sharks, i.e. a spiral staircase inside the (otherwise 

 shortened) intestine, which serves to increase the surface for ab- 

 sorbing the digested food. Thus of a type of animal that left the 

 stage millions of years ago we can say that it fed largely on cuttle- 



