MEDIAN EYES IN EXTINCT VERTEBRATES 



333 



over in the more primitive types of living reptiles, such as Sphenodon, the 

 organ is found to be more highly differentiated than in the less primitive ; 

 but in all living reptiles, including Sphenodon, the pineal eye frequently shows 

 signs of degeneration, such as the development of pigment in the lens or 

 the frequent absence of its nerve in the adult animal, and judging from 

 the large size of the pineal canal in some extinct amphibia and reptiles 

 the organ in living species is relatively extremely small. 



This difference in size of the pineal foramen suggests that in those 

 extinct animals in which the foramen was large, the organ itself may have 

 been not only larger but also more highly differentiated, and may even 

 have served as a visual organ, as contrasted with a light-perceiving organ. 



Fig. 234. — Restoration of Skull of Dinichthys intermedius, viewed from 

 in Front and showing the Position of the Pineal Plate. (After Anatol 

 Heintz.) 



But in many of the fossil skulls of fishes, amphibians, and reptiles the 

 foramen is closed or small and in some completely absent, indicating that 

 the organ had become vestigial in these animals at a very early period of 

 their evolution. 



The pineal impression in certain of the ancient Ostracoderm fishes 

 is, as we have already mentioned, seen only on the inner surface of the 

 pineal plate, the outer or superficial surface in some cases being smooth 

 (Fig. 234), while in others an external impression is also visible, but it 

 is less deep than the pit found on the intracranial surface. These fossil 

 markings correspond closely with the conditions which are found in 

 many living examples of the more primitive types of cartilaginous and 

 bony fishes, such as the spiny dogfish (Spinax niger) (Fig. 49) and the 



