CHAPTER 23 



THE GEOLOGICAL EVIDENCE OF THE EXISTENCE 

 OF MEDIAN EYES IN EXTINCT VERTEBRATES 



The anatomical evidence of the existence of both median and lateral 

 eyes in the extinct order of Palaeozoic fishes known as the Ostracodermata, 

 which lived in the Silurian and Devonian periods, is of great value not 

 only with reference to the antiquity of the vertebrate eyes but also with 

 respect to the constancy of the pattern formed by the relative positions 

 of the cavities and impressions on the head-shields which lodged the 

 sense-organs, namely : the small pineal impression in the centre ; the 

 orbital cavities for the lateral eyes on each side ; the narial and hypo- 

 physeal apertures in front and the smooth " glabellar plate " forming 

 the roof of the cranial cavity which contained the brain behind l (Fig. 228). 

 Moreover, palaeontology affords very strong evidence of the bilateral 

 origin and nature of the pineal body, e.g. the existence of two impressions 

 placed side by side on the outer surface of the pineal plate of Pholidosteus 

 and Rhinosteus, recorded by Stensio (Fig. 229) ; the two pits also placed 

 side by side but on the inner surface of the pineal plate of Titanichthys, 

 described by E. S. Woodward (Fig. 230), along with the heart-shaped 

 foramen figured by E. S. Hill on the dorsal aspect of the skull of Dipnor- 

 hynchus (Fig. 140, Chap. 18, p. 200) and the similar heart-shaped pit on 

 the intracranial aspect of the pineal plate of Dinichthys intermedius shown 

 in the drawing reproduced from Adolf Heintz (Fig. 231). All these 

 examples tend to confirm the similar conclusions with respect to the 

 bilateral origin of the pineal which have been founded on the comparative 

 anatomy and comparative embryology of living species and advocated by 

 Cameron (pp. 292, 294), Dendy (p. 245), Gaskell (p. 187), Hill (p. 218), 

 Kingsbury (p. 215), Locy (p. 202), and others. Further, the palasontological 

 evidence of the position and relations of the orbital cavities and pineal im- 

 pressions in fossil vertebrates, more especially in the head-shields of the 

 Ostracodermata, taken along with other evidence of a more general 

 character seems to indicate the existence at a very early period of a common 

 ancestral stock from which arose the prevertebrate stem of these Palaeozoic 



1 The glabellar plate corresponds to the " dorsal electric field, a slightly depressed 

 spear-shaped area, which Stensio considers may have lodged a dorsal median electric organ. 



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